sort: fix unportable cast of unsigned char * -> char *

sort: fix unportable cast of unsigned char * -> char *

* src/sort.c (fold_toupper): Change this back from char to
unsigned char, fixing a portability issue introduced in commit
59e2e55d0f154a388adc9bac37d2b45f2ba971f8 dated February 26, as the
C Standard doesn't let you convert from unsigned char * to char *
without a cast, and the (in theory more portable) style here is to
convert char values, not pointer values.
(getmonth): Convert char to unsigned char when needed for
comparison.