locale.c: Revamp finding if locale is UTF-8

locale.c: Revamp finding if locale is UTF-8

This changes how this functionality works for the LC_CTYPE locale.  On
systems that have nl_langinfo() one can get a "definitive" answer from
just that.  Otherwise (or if that doesn't return properly) one can use
mbtowc() to check if the UTF-8 byte sequence for the Unicode REPLACEMENT
CHARACTER actually is considered to be that code point.  This is also
"definitive".  If the maximum byte string length for a character is too
short to handle all Unicode UTF-8, we know without further checking that
this isn't a UTF-8 locale, so can avoid the mbtowc check.

It turns out, from testing, that some locales are labelled UTF-8 by
nl_langinfo even though they depart from that at times.  Similarly for
mbtowc().  Perl assumes that a locale doesn't depart from this, and uses
its internal rules that it knows are UTF-8.  A future commit will warn
when this happens.