First the two that add something (ADD, ENTER), then the three that
delete something (BACK, DEL, JOIN), and then the one that changes
something (REPLACE).  Then the SPLITs, CUT, PASTE, and INSERT, and
then the INDENTs and COMMENTs, when they exist.

For cuts, pastes, and inserts, the lines have already been renumbered;
for indents, comments, and replacements, the line numbers cannot have
changed.  (And anyway, variable 'f' is not set for those cases.)

Only when lines get split (ENTER) or fused together (JOIN) do the later
lines need to be renumbered.  This mirrors what is done for do_redo().

The 'f' variable is used only in the ADD, BACK, DEL, ENTER, JOIN, and
REPLACE undo/redo cases.  So, avoid making a somewhat costly call when
it is entirely superfluous.  Rearrange the undo types to make checking
for the above six types easier.

This fixes https://savannah.gnu.org/bugs/?53272.

On a Linux console, ^/ produces Backspace, making it unrebindable.

This fixes https://savannah.gnu.org/bugs/?53248.

It is not a problem to say that ^X closes the current buffer
even when nano is compiled without multibuffer support.

This assumes that all movement functions are located together,
starting with to_first_line() and ending with do_right().

This fixes https://savannah.gnu.org/bugs/?53195

.
Reported-by: Brand Huntsman <alpha@qzx.com>

As of commit c79fe1a1, the more readable ^/ has replaced ^_ as the
advertised shortcut for Go-To-Line -- color it too as valid.

To prevent getting kicked out of it.

This fixes https://savannah.gnu.org/bugs/?53234.

The implant() function itself cannot be bound to anything, so it
is not in the linked list of functions -- trying to find it would
lead to a NULL pointer, and thus to a segfault.

This fixes https://savannah.gnu.org/bugs/?53233.

Commit 0cf455bc introduced the typo.

And remove a detail that is untrue when 'set selectedcolor' is used.

This fixes https://savannah.gnu.org/bugs/?53225.

Instead of always calling sctofunc(), it is now only called when
in view mode OR when (after the keystroke's function has been run)
no need for a refresh has been established yet.

A shortcut's function must be among the existing ones, otherwise
nano's code is fundamentally broken.

Commit bb667beb removed do_gotolinecolumn_void() from the MWHEREIS
menu (replacing it with the empty flip_goto() function), so there
is no longer any need to check for this one special case.

A shortcut's function can never be NULL.

It is only ever set and never referenced.

Exclude the confusing, pleonastic 'sc' abbreviation from their names.

The new option 'set errorcolor' allows the user to specify the color
combination for the status bar when an error message is displayed.

This fixes https://savannah.gnu.org/bugs/?53188

.
Reported-by: Ken Tyler <kent@werple.net.au>

The slash is easier to read than the underscore (which almost
disappears at the bottom of the screen), and easier to type
(no Shift needed on a US keyboard), and it kind of harmonizes
with the ^\ for Replace and the M-/ for End-of-buffer.

See the discussion on the mailing list:
https://lists.gnu.org/archive/html/nano-devel/2018-01/msg00022.html
https://lists.gnu.org/archive/html/nano-devel/2018-02/msg00090.html

Signed-off-by: Tom Levy <tomlevy93@gmail.com>