Fix char2wchar/wchar2char to support collations properly.

These functions should take a pg_locale_t, not a collation OID, and should
call mbstowcs_l/wcstombs_l where available.  Where those functions are not
available, temporarily select the correct locale with uselocale().

This change removes the bogus assumption that all locales selectable in
a given database have the same wide-character conversion method; in
particular, the collate.linux.utf8 regression test now passes with
LC_CTYPE=C, so long as the database encoding is UTF8.

I decided to move the char2wchar/wchar2char functions out of mbutils.c and
into pg_locale.c, because they work on wchar_t not pg_wchar_t and thus
don't really belong with the mbutils.c functions.  Keeping them where they
were would have required importing pg_locale_t into pg_wchar.h somehow,
which did not seem like a good plan.