sort: add a --debug option to highlight key extents

sort: add a --debug option to highlight key extents

* src/sort (usage): Add description for --debug.
(write_bytes): Pass a line structure so it can subsequently
be passed to compare to highlight the keys when in debug mode.
Also transform TAB and NUL characters written to stdout so
that the highlighting in debug mode aligns correctly.
(human_numcompare): Pass an "endptr" so we can record the extent
of the number matched.
(general_numcompare): Likewise.
(find_unit_order): Likewise.
(getmonth): Likewise.
(numcompare): Likewise.  Note we reuse find_unit_order() for this,
which is a good enough approximation, and means we don't need to
change the strnumcmp() interface.
(check_mixed_SI_IEC): Return whether iec_present, so that can be
used to set the "endptr" in find_unit_order.  Also make the key
parameter optional, which will be the case from numcompare().
(count_tabs): A new function to determine how much to adjust
the mbswidth() values by (TABs don't have a width).
(mark_key): A new function to output the key highlighting to stdout.
(debug_key): A new function to determine the offset and width
of the key highlighting.
(key_compare): Pass the show_debug parameter so the key highlighting
is only displayed when explicitly called.  For each key type, set
the length (lena) and whether leading blanks are auto skipped (skipb)
which are then used by debug_key() to highlight the portion of the
key used in the comparison.
(compare): Pass the show_debug parameter so the key highlighting
is only displayed when explicitly called.  Call debug_key() to
highlight the last resort comparison.
(check): Output highlighting for disorder line to stdout.
(main): Process the --debug option and make it mutually exlusive
with the -o option as I don't see it useful there, even potentially
harmful if someone left a --debug in by mistake when updating a file.
Also restricting debug output to stdout, simplifies the logic
for dealing with temporary files.
* doc/coreutils.texi (sort invocation): Describe the --debug option,
and reference it from the --key description.
* tests/misc/sort-debug-keys: A new test for highlighting keys.
* tests/Makefile.am: Reference the new test.
* NEWS: Mention the new feature.