sort: tune and refactor --debug code, and fix minor underlining bug

sort: tune and refactor --debug code, and fix minor underlining bug

Formerly, the 'compare' function and some of its subroutines had a
debugging flag, which caused them to output underlines.  This
change refactors the code so that debugging output is
more-separated from the actual sorting.  In the process, the
change fixes a minor error in the debugging output.  The change
shortens the source code and executable size a tad, and improves
CPU performance by 2.4% on my platform with a simple benchmark (C
locale, line sorting, no debug).

* src/sort.c (long_double, strtold): Move back to prelude, since
they're now used by multiple functions again.
(unit_order): Move to file scope, since it's now used by two functions.
(find_unit_order, human_numcompare, numcompare, general_numcompare):
Remove endptr parameter.  All callers changed.
(human_numcompare): Args are now const pointers.
(getmonth): Endptr is now non-const.
(key_numeric): Move up, since it's needed earlier.
(debug_key): Take a line and a key as argument, instead of having
the caller figure out where the field is.
(debug_line): New function.
(keycompare, compare): Omit debug parameter; debug output now done
elsewhere.  All callers changed.
(write_line): Renamed from write_bytes; all callers changed.
Use debug_line (not 'compare') to output debug info.
Use a slightly faster check for whether output file is stdout.
(check): Don't do debugging output; it's not that useful here,
and it confuses the code.
(main): Check for incompatibility between -c and --debug.
Use standard diagnostic for incompatible options.
* tests/misc/sort-debug-keys: Fix test case: "--Mi-1" is not
a number, so its first character should not be underlined when
debugging a numeric sort.