locale.c: Extract out, fix, expand fcn to see if a locale is utf8

There was buggy code to see if the start-up locale is UTF-8.  This
commit extracts it into a separate function.

The bugs involved looking at the name of the locale to see if that
implies a UTF-8 name.  Prior to this commit, it looked at the
beginning of the locale name, whereas in reality, it is at the end, as
in "fr_FR.UTF8".

Also, it didn't look for the documented Windows name for UTF-8 locales
on those platforms.

The function is expanded to have an input category to find the utf8ness
of.  Thus it now works on any non-LC_ALL category, not just LC_CTYPE.

It is possible for categories to be in different locales, so that
LC_CTYPE is in a UTF-8 locale, and LC_NUMERIC isn't.  For the purposes
of PERL_UNICODE, the most applicable category is LC_CTYPE, so that is
the one used.