sort: fix issues with month sorting in some locales

sort: fix issues with month sorting in some locales

* src/sort.c (char fold_toupper[]): Change to unsigned
so as the correct comparisons are made in getmonth().
This fixes unibyte locales where abbreviated months
have characters that are > 0x7F, but it also works for
multibyte locales with the caveat that multibyte characters
are matched case sensitively.
With this change, the following example sorts correctly:
  $ echo -e "1 márta\n2 Feabhra" | LANG=ga_IE.utf8 sort -k2,2M
  2 Feabhra
  1 márta
* src/sort.c (inittables): Since we ignore blanks around months
in the input, don't include them when they're present in the locale.
With this change, the following example sorts correctly:
  $ echo -e "1 2月\n2 1月" | LANG=ja_JP.utf8 sort -k2,2M
  2 1月
  1 2月
* tests/misc/sort-month: A new test to exercise the above cases.
* tests/Makefile.am: Reference the new test.
* NEWS: Mention the fix.