Make nano recognize the escape sequences for Ctrl-Home, Ctrl-End,
Shift-Ctrl-Home, and Shift-Ctrl-End on xterm, rxvt, and Eterm.

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.

When Enter is pressed while the cursor is exactly on the current
indent width, remove the blank characters on that line to avoid
creating a line that consists only of trailing whitespace.

(When Enter is pressed somewhere in the middle of the blanks,
however, the whitespace is preserved.)
Suggested-by: Florian Zeitz <florob@babelmonkeys.de>

When a multi-column character straddles a chunk boundary, and the
preferred column (placewewant) for the cursor is zero, cheat: show
the cursor not where the character starts but on the beginning of
the next row.  This makes the cursor move smoothly in the leftmost
column of the screen when using <Up> and <Down> and such, instead
of jumping around.

In this way the scrolling logic won't get confused and the screen
will scroll properly when stepping beyond the top or bottom row.

This fixes https://savannah.gnu.org/bugs/?50687.

This fixes https://savannah.gnu.org/bugs/?50741.

If we're using the bundled gnulib regex module, then assume REG_ENHANCED
is not supported (since gnulib doesn't support it).

This fixes https://savannah.gnu.org/bugs/?50714.

When UTF-8 is available, use actual arrows instead of untranslated words
to indicate the cursor keys.  This was already done for the combinations
with Ctrl but not yet for the plain cursor keys.

The unshifted shortcuts are easier to type, and also less confusing in
my eyes.  Putting them first means they get shown in the help lines,
and get listed first in the ^G help text.

(I would also like to put ^- first instead of ^_ (because the latter
is hard to see when using the default inverse video for shortcuts),
but on several terminal emulators Ctrl+- reduces the font size.)

Making ^Up and ^Down go to top and bottom row in the file browser
complements and completes the behavior of ^Left and ^Right.

Also, don't bother copying the found bracket -- just compare it in situ
and thus elide a variable.

Verify at startup that the number is not too small.

Rename some variables in the process, and remove two cluttering asserts.

(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.

If we're using the bundled gnulib regex module, then assume word boundary
support is available to avoid issues with the regcomp test.  This also
unifies the different code paths a bit.

This fixes https://savannah.gnu.org/bugs/?50705.

If edittop is partially offscreen before we scroll, and it gets
scrolled more offscreen, we do need to compensate for the chunks
between firstcolumn and leftedge -- that is: the chunks between
the top row and the cursor row.

This fixes https://savannah.gnu.org/bugs/?50691.

When a two-column character cannot be shown because it straddles the
boundary between two chunks of a line, show the '>' placeholder for
its left "half", and '<' for its right "half".

This mitigates https://savannah.gnu.org/bugs/?49440.

Remove some things we're never going to do: allowing to replace
newlines, a regression framework, characterset conversions, or
jumping to various screen lines.

The leftedge variable is a column number, not an x position.

The number of rows to draw shouldn't be compensated for the chunks
of edittop that are before firstcolumn, because they are offscreen.

This completes the fix for https://savannah.gnu.org/bugs/?50621.

There is no need to always increase nrows by 1 or 2 -- an increase
of 1 is only needed when the line that borders on the scrolled region
needs to redrawn too: when this line was horizontally scrolled or when
the mark is on.

This fixes https://savannah.gnu.org/bugs/?50621

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

The change in commit 244a503d for the <Up> and <Down> keys
needs to be done for <PageUp> and <PageDown> too.

The complementary test on current_y should only be done when doing
a scroll-only, because only then the prior line can be offscreen.

This fixes https://savannah.gnu.org/bugs/?50658

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

When scrolling backward, it is not just the bottom line of the screen
that doesn't need to be redrawn: also the line /before/ the top line
doesn't need a redraw.  Mutatis mutandis for scrolling forward.

This fixes https://savannah.gnu.org/bugs/?50657.