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.

This fixes https://savannah.gnu.org/bugs/?51918.

And use these constants in another context too.

When moving the cursor to the top-left corner, it is not necessary
to compute leftedge because firstcolumn IS the relevant leftedge.
Reported-by: Jordi Mallach <jordi@debian.org>

Don't skip rows that contain something when moving up, and do put
the cursor in the desired column whenever possible when moving down.

This fixes http://savannah.gnu.org/bugs/?51827
and fixes http://savannah.gnu.org/bugs/?51828.

This fixes http://savannah.gnu.org/bugs/?51821.

It's not yet good enough: when proper_x() pushes the index forward,
maybe it should also increase leftedge?  Or go_forward_chunks()?

This helps to fix https://savannah.gnu.org/bugs/?51787

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

This fixes https://savannah.gnu.org/bugs/?51809.

When the screen is narrower than a tab is wide, the cursor does not need
to be on the first/last line of the window before an <Up>/<Down> could
need to scroll the screen.

This fixes http://savannah.gnu.org/bugs/?51776

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

This fixes https://savannah.gnu.org/bugs/?51802.

This is a temporary fix.  The resultant movement is erratic and
not what is desired (which is: to stay in the same screen column
as much as possible).

Also, move home to the first character after the tab if the current
chunk starts with a partial tab.

This fixes https://savannah.gnu.org/bugs/?51800

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

This should have been added and amended in the previous commit --
my mistake.

Also bind 'N' to a backward re-search, while retaining 'n' for a
forward one,  This matches the functionality in the help viewer.

Check the column number, not the character index, when suppressing the
line number on chunks other than the first, since the index can be zero
when in fact we're on a later chunk.

This fixes https://savannah.gnu.org/bugs/index.php?51790.

This fixes https://savannah.gnu.org/bugs/?51777

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

Using do_up() and do_end() when the user types <Left> at the start of
a line, and do_down() and do_home() when typing <Right> at line's end
can be problematic when tabs are wider than the screen, because those
functions convert indexes to columns and back again twice, thus causing
inaccuracies.  Therefore, simply adjust current and current_x directly,
and then redraw the screen.

This fixes https://savannah.gnu.org/bugs/index.php?51778.

Regardless of whether the row start is at a multiple of the tab size.

This completes the fix for https://savannah.gnu.org/bugs/?51669.

This fixes https://savannah.gnu.org/bugs/?51770,
and fixes https://savannah.gnu.org/bugs/?51768

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

This is a partial fix for https://savannah.gnu.org/bugs/?51770

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

Also rename a variable, to match the saving routine.

Also reshuffle an initialization and drop two useless asserts.