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

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

Call something a buffer when it refers to a linked list of linestructs,
and call something a linestruct when it is a struct that describes a
single line.

Trim a superfluous assert too.

File formats, appending and prepending, and backups are not available
when --enable-tiny is used, so prevent all relevant pieces of code from
getting compiled.  And correct two misspelled ENABLE_TINY to NANO_TINY.

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

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.

Verify at startup that the number is not too small.

(We still leak the backupname in that case, but I can't be bothered
with that now.)

Also elide a variable, trim some comments, and rewrap some lines.

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.

The "./" is a shorthand for "current working directory".
It is better to specify it, because it differs from what
Pico does: reading always from the user's home directory
no matter where the editor was started.

Only use the "from" thing when an operating directory is in effect,
because /only/ then the indicated directory can be something other
than "./".

Also, make it so that there is no space before the colon.

We want to be able to scroll the line at edittop partially off the
screen.  For this to be possible, the new variable firstcolumn stores
the starting column of the viewport -- the starting column in the line
that edittop points to.

Since firstcolumn is used by go_back_chunks() and go_forward_chunks(),
it can't be completely #ifdefed out when NANO_TINY is set, but outside
of softwrap mode it should always be zero.

Currently firstcolumn is initialized to zero, reset to zero when
toggling softwrap mode off, and reset to zero when switching buffers
while softwrap mode is off.  It's otherwise unused, but its uses are
forthcoming.

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.

Use less_than_a_screenful() again, just as in do_uncut_text().

Most of these variables are freed moments later -- reallocating
them is thus a waste of time.

Windows doesn't have *nix style account databases.

With read_file() revamped, it now uses partition_filestruct() indirectly
via ingraft_buffer(), so we can't use partition_filestruct() to replace
marked text in the alternate spell checker anymore without segfaulting.

Add the new function replace_marked_buffer() to accomplish this instead.
Based on replace_buffer(), it uses extract_buffer() to throw away the
marked un-spell-checked text, and then uses read_file() to insert the
spell-checked text at the position where the mark was.

Accordingly, remove unneeded partitioning and related stuff from
do_alt_speller().  Besides pasting the file into the buffer at
current[current_x], ingraft_buffer() also deals with renumbering,
updating totsize, and handling a magicline, so do_alt_speller()
doesn't need to do those anymore.

With read_file() revamped to handle inserting a file at the current
position, partitioning the buffer is no longer needed.

Move buffer handling and '\r' stripping from read_line() to read_file(),
so that the file gets its format determined and gets stored in its own
buffer entirely in one function.  Then use ingraft_buffer() to insert
this new buffer into the current one.

In addition to pasting the file at current[current_x], ingraft_buffer()
also deals with renumbering, the updating of totsize, and the handling
of a magicline, so read_file() doesn't need to do those anymore.

Note that all this makes read_file() depend on the position of
current[current_x] to know where to insert the file.  Accordingly,
set current_x to zero in initialize_buffer_text() instead of in
make_new_buffer(), so that replace_buffer() keeps working properly.

This fixes https://savannah.gnu.org/bugs/?49892.

This fixes https://savannah.gnu.org/bugs/?49994

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

Instead of setting openfile->current_y (and wrongly so), just call
reset_cursor() to recompute current_y and place the cursor on that
line (if it is not offscreen).

The conditionalizing saved negligible amounts of space, of memory, and
of speed.

This makes nano's cursor behavior consistent across 1) typing text by
hand; 2) pasting in text with ^U; 3) inserting text from a file; and
4) redoing with M-E that same typing or pasting or inserting.

This fixes https://savannah.gnu.org/bugs/?49968.

The warning is wrong when the user has just saved a buffer under a
new name.  And when --quickblank is used, the warning most likely
gets cleared off before it is seen, and the user would just hear
the beep and be left wondering what happened.

This avoids https://savannah.gnu.org/bugs/?49875.

When a given file is, for example, unreadable or unwritable, the
error message should use ^J instead of ^@ in its name.

In path names and file names, 0x0A means an embedded newline and
should be shown as ^J, but in anything related to the file's data,
0x0A is an encoded NUL and should be displayed as ^@.

So... switch mode at the two main entry points into the "file system"
(reading in a file, and writing out a file), and also when drawing the
titlebar.  Switch back to the default mode in the main loop.

This fixes https://savannah.gnu.org/bugs/?49893.

Decode 0x0A bytes to 0x00 when saving the search history, and encode
them again when reading the file back in, to prevent nano from hanging
or aborting when encountering 0x00 on a line by itself.

After updating the color palette, the corresponding new window content
should be drawn immediately, before some other part of the code calls
doupdate(), to prevent the old content being shown in the new colors.

This fixes https://savannah.gnu.org/bugs/?49912

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Reported-by: Mike Frysinger <vapier@gentoo.org>