spotlight() now displays softwrapped lines chunk by chunk instead of all
at once.  Since softwrapped lines are no longer of constant width, the
latter approach would fail if softwrapping breaks the spotlighted text.

Instead of taking a string, spotlight() now takes the starting and ending
columns of that string.  Also, its handling of softwrapped lines is now
split off into a separate function, spotlight_softwrapped().

Add the new function actual_last_column() to accomplish this.

get_chunk_row() replaces the formula "column / editwincols".

get_chunk_leftedge() replaces "(column / editwincols) * editwincols".

get_last_chunk_row() replaces "strlenpt() / editwincols".

get_last_chunk_leftedge() replaces "(strlenpt() / editwincols) * editwincols".

This prepares us for any changes in those formulas, and for more such
functions later.

The new function find_softwrap_breakpoint() returns the column number
of the last position in screen range where we can wrap the given text
without breaking a two-column character in half (as was done until now).
The returned column number is the leftedge of the next softwrapped chunk.

If the end of the text is reached while searching for a wrapping point,
the parameter end_of_line is set to TRUE.

The new function get_chunk() uses find_softwrap_breakpoint() to find the
row and leftedge corresponding to a given column of a given line.

Achieve this by making the suppression flag global, so that we can
just reset it instead of making an improper call of do_cursorpos().

This fixes the secondary part of https://savannah.gnu.org/bugs/?51134.

When spotlighting the string to be replaced, placewewant isn't valid,
so tell place_the_cursor() to ignore its value to avoid the cursor
getting mistakenly placed at the beginning of the next row.

This fixes https://savannah.gnu.org/bugs/?50997

.
Reported-by: David Lawrence Ramsey <pooka109@gmail.com>

Also, allow rebinding the word and block jumping functions
in the tiny version when nanorc files are reenabled.

Also, allow Ctrl+Left/Right with --enable-tiny --enable-browser.

Also remove a superfluous check for 'do_replace' -- it never gets
bound in the Search/Replace menus.

When configured with --enable-tiny --enable-multibuffer,
/do/ include the new-buffer toggle in the Read-File menu.

And hide the cursor again as soon as the user scrolls.
Achieve this through making the 'didfind' variable global.

Also, remove a superfluous call of wnoutrefresh(), as bottombars()
already does that.

This fixes https://savannah.gnu.org/bugs/?50918

.
Reported-by: David Lawrence Ramsey <pooka109@gmail.com>

Also, trim some comments and avoid an unused-variable warning.

Also, reshuffle two initializations to go sit with the other ones.

Things have morphed over time and display_buffer() no longer actually
displays the buffer -- it just displays the title bar, precalculates
the multiline color info, and schedules a refresh of the edit window.

This avoids https://savannah.gnu.org/bugs/?49912 while at the same time
avoiding to draw the edit window twice in a row -- the first drawing
would use a wrong margin, which results in a visible and irritating
shift left or right of the content upon the second drawing.

This fixes https://savannah.gnu.org/bugs/?50877.

And one-letter variables I cannot "see" -- they are too small.

An iterator should not be called "start_col", because it is only the
starting column at the very beginning.

Also, start_col (after the rename) can never be /larger/ than column.

And in the process transform the token DISABLE_HELP to ENABLE_HELP.

Allow the user to search in a help text with ^W and M-W.

Achieve this by not writing the help text directly to the screen
but first writing it to a temporary file and then opening this file
in a new buffer, and treating it specially: the normal file-reading
feedback is suppressed, the titlebar shows the headline of the text,
the cursor is hidden, and the menu is limited to just the up and down
movements and searching.

This fulfills https://savannah.gnu.org/bugs/?28994

.
Signed-off-by: Rishabh Dave <rishabhddave@gmail.com>

Because nothing gets reset to zero or to some initial value.

These functions are used only in the file in which they are declared,
and they are declared before they are used.

The interval 2013-2017 for the Free Software Foundation is valid
because in those years there were releases with changes by either
Chris or David, and the GNU maintainers guide advises to mention
a new year in all files of a package, not just in the ones that
actually changed, and be done with it for the rest of the year.

On some terminal emulators, Ctrl+Home and Ctrl+End produce special
keycodes, distinct from plain Home and End.  Make the users of those
emulators (and of the Linux console) glad by making ^Home and ^End
do the obvious thing, and the combinations with Shift too.

Verify at startup that the number is not too small.

Use futimens() instead of utime() to change the timestamps on a backup
file.  Otherwise, a non-privileged user could create an arbitrary symlink
with the name of the backup file and in this way fool a privileged user
to call utime() on the attacker-chosen file.

Import the relevant gnulib module to make sure futimens() is available.

If the number of columns in the edit window changes (which currently
only happens in two places: in regenerate_screen(), called when the
window is resized; and in main(), when line numbering mode is toggled),
the display will break if we're in softwrap mode and firstcolumn is
nonzero.  This is because the column width of softwrapped chunks has
changed, and firstcolumn is no longer the starting column of a chunk,
an assumption that all code using firstcolumn relies on.

To fix this problem, add a new function, ensure_firstcolumn_is_aligned(),
to adjust firstcolumn to the starting column of the chunk it's on, and
use it when the number of columns in the edit window changes.

(Note that this function uses the simplest possible fix, and could
probably be made more sophisticated.)

The new function, update_softwrapped_line(), is called from inside
update_line() when softwrap mode is on, so that existing calls remain
unchanged.  It takes no index, instead displaying edittop from column
firstcolumn, and all other lines from column zero.

If current is on edittop, it's displayed using the edittop rules, but
this is not a problem: if current[current_x] is above edittop at column
firstcolumn, it's offscreen, and that should be handled before calling
update_line() anyway.

Together with the preceding bunch of changes,
this fixes https://savannah.gnu.org/bugs/?47667.

Since all lines can be partially scrolled off the screen now
(except for the top line of the edit window, which is forthcoming),
ensure_line_is_visible() is no longer needed.

Since all lines can be partially scrolled off the screen now
(except for edittop, which is forthcoming), the maxlines global
variable and its computation mechanism are no longer needed.

Add the parameter be_clever to both functions.  When be_clever is FALSE,
smart home and dynamic home are disabled in do_home(), and dynamic end is
disabled in do_end(), so that these functions only move to the beginning
or end of the current line or chunk.

This simple home and end functionality is needed to improve do_left()
and do_right()'s horizontal behavior with softwrapped chunks, which is
forthcoming.

These improvements will eventually make do_home() and do_end() take
parameters.  Since the global function lists can hold only functions
without parameters, preemptively add do_home_void() and do_end_void(),
and make the global function lists use them.