Adjust commentary in regc_pg_locale.c to remove mention of the possibility
of not having <wctype.h> functions, since we no longer consider that.

Eliminate duplicate code in wparser_def.c by generalizing the p_iswhat
macro to take a parameter saying what to return for non-ASCII chars
in C locale.  (That's not really a consequence of the
USE_WIDE_UPPER_LOWER-ectomy, but I noticed it while doing that.)