This is modeled after the comment/uncomment code.  Each line's
individual indent is saved in the string array of the undo
group structure.

This fixes http://savannah.gnu.org/bugs/?46860.

The function does not contain any comment-specific code, so it can
be used to handle any kind of multiline undo item.

Also, extend the undo group structure to contain an array of strings,
one for each line in the group.  When indent/unindent is hooked up to
the undo/redo code, this will allow the latter to restore the exact
original indents.

Also, remove an unneeded null_at() -- the null byte has been copied,
and reallocating the string would recover very little memory.

Also, call charmove() without using the & operator.

This is modeled after the undo/redo code for commenting.  do_indent() now
calls indent_a_line() on each line it covers.  The latter function will
eventually be used by the undo/redo code.

Also reshuffle an initialization and drop two useless asserts.

The preceding 'for' loop has already established that all the lines are
either unindentable or composed of only blanks.  So if now a line cannot
be unindented, it means it is fully blank, so it can be simply skipped.

This fixes https://savannah.gnu.org/bugs/?51760

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

Also, trim or improve some comments.

Also, only unindent when all selected lines can be unindented,
which means that the relative indentation will be preserved.
For this purpose, it ignores empty lines and lines consisting
of only whitespace.

When unindenting is not possible, a message is shown.

This fixes https://savannah.gnu.org/bugs/?47684.

Instead of inserting the extra whitespace after the current indentation
of a line, add it to the start of the line.  This causes a fixed amount
of visual whitespace to be added regardless of whether --tabstospaces
is used or not.

This fixes http://savannah.gnu.org/bugs/?51438,
and its ancestor https://savannah.gnu.org/bugs/?51408.

Without them, nano still compiles for me, with everything enabled,
even when using --enable-debug, --enable-utf8, and --with-slang.

Assume that a linter line that contains an actual error message or
warning includes a colon followed by a space -- something that that
opening message from a modern gcc lacks.

Human column numbers are one-based, whereas x positions are zero-based.
Converting from the one to the other involves subtracting one.  But when
the linter message does not provide a column number, the latter defaults
to zero.  Catch that case and change the number to one.

This fixes https://savannah.gnu.org/bugs/?51550.

The charalloc() macro does exactly this.

Commit 28beb3f9 added the 'forreal' parameter to prevent spotlight() from
placing the cursor wrongly due to an invalid placewewant.  However, since
the variable-width softwrap overhaul (specifically, since commit 8490f4ac),
place_the_cursor() no longer checks placewewant, so the parameter is no
longer needed.

Furthermore, dropping 'forreal' and thus always setting current_y won't
affect the operation of spotlight(), since the only functions that use
spotlight() (do_replace_loop() and do_int_spell_fix()) both call
edit_refresh() beforehand, which means that current_y will already
have been set to the value it will be set to again.

The help lines need to be redrawn one step after a justification
(whether it has been undone or not, to replace "Unjustify" with
"Uncut" again for ^U), and after switching buffers (to update a
possibly changed tag for ^T).

This fixes https://savannah.gnu.org/bugs/?51455.

Make them reflect the split into two functions, the direct use of
tabsize, and the difference in behavior between indent and unindent.

Since all indentation and unindentation is by a tab, or by a tab's
worth of spaces, use tabsize directly.

Also remove one unneeded blank line.

The parameter 'cols', that indicates how many columns to indent or
unindent, is changed to be always positive, so the check for being
negative can go.  And it could never be zero anyway.

This is the first step toward splitting them into two dedicated
functions, instead of letting do_indent() handle both cases.

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().

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.

If the position of the cursor changes horizontally,
placewewant should change with it.

This fixes https://savannah.gnu.org/bugs/?51407

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