Trying out native UTF-8 locales on Windows

Here's a very short patch to experiment with the idea of using
Windows' native UTF-8 support when possible, ie when using
"en-US.UTF-8" in a UTF-8 database.  Otherwise it continues to use the
special Windows-only wchar_t conversion that allows for locales with
non-matching locales, ie the reason you're allowed to use
"English_United States.1252" in a UTF-8 database on that OS, something
we wouldn't allow on Unix.

As I understand it, that mechanism dates from the pre-Windows 10 era
when it had no .UTF-8 locales but users wanted or needed to use UTF-8
databases.  I think some locales used encodings that we don't even
support as server encodings, eg SJIS in Japan, so that was a
workaround.  I assume you could use "ja-JP.UTF-8" these days.

CI tells me it compiles and passes, but I am not a Windows person, I'm
primarily interested in code cleanup and removing weird platform
differences.  I wonder if someone directly interested in Windows would
like to experiment with this and report whether (1) it works as
expected and (2) "en-US.UTF-8" loses performance compared to "en-US"
(which I guess uses WIN1252 encoding and triggers the conversion
path?), and similarly for other locale pairs you might be interested
in?

It's possible that strcoll_l() internally converts the whole string to
wchar_t internally anyway, in which case it might turn out to be
marginally slower.  We often have to copy the char strings up front
ourselves in the regular path strcoll_l() path in order to
null-terminate them, something that is skipped in the wchar_t
conversion path that combines widening with null-termination in one
step.  Not sure if that'd kill the idea, but it'd at least be nice to
know if we might eventually be able to drop the special code paths and
strange configuration possibilities compared to Unix, and use it in
less performance critical paths.   At the very least, the comments are
wrong...