To avoid an ncurses hiccup (miscoloring) when running on musl.

This fixes https://savannah.gnu.org/bugs/?50787

.
Reported-by: Avi Halachmi <avihpit@yahoo.com>

The cursor needs to be placed in its proper spot in the edit window
/only/ when nano is about to accept input from the user and needs to
show where this input will go.

(This might cause a scrolling issue to appear, because reset_cursor()
does not just place the cursor, it also recomputes current_y, which
is used in several places to determine whether and how much to scroll.
If it so happens, we'll deal with that fallout later.)

This also groups ^I and ^M together, and cutwordleft and cutwordright
(when they are bound).  It furthermore makes that less pairs are now
mixed and instead consist of either two Ctrl or two Meta combos.  In
short: it looks better in the default config.  The only sacrifice is
that Verbatim is now split off from the other "inserting" keys.

This fixes https://savannah.gnu.org/bugs/?47131.

When the user chooses not to open a file that some message refers to,
remove all messages for that file from the linting results, so the user
does not get asked about that same file again.

This fixes https://savannah.gnu.org/bugs/?47130.

Replace them by a single subtraction.

When UTF-8 is available, use actual arrows instead of untranslated words
to indicate the cursor keys in the search history, as is done elsewhere.

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.

To make dynamic Home and End work properly when double-width characters
straddle a chunk boundary, use the spot where the cursor is really shown
instead of the "actual x" position of the current character, because the
latter might be on the preceding row.

This fixes https://savannah.gnu.org/bugs/?50737.

Also, M-A is far more mnemonic for setting the mark than ^6 or ^^,
so it's better to try and teach the user that.

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.

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 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.