Properly handle systems with crippled locales

Properly handle systems with crippled locales

Some systems fake their locales, so that they pretend to accept a locale
change, but they either do nothing, making everything the C locale, or
on some systems there is a a second locale "C-UTF-8" that can be
switched to.  Configure probes have been added to find such systems, and
this commit changes to use the results of these probes, so that we don't
try looking for other locales (any names we came up with would be
accepted as valid, but don't work, and tests were failing as a result).

Anything running the musl library fits, as does OpenBSD and its kin, as
they view locales as security risks.  This commit allows us to take out
some code that was looking for particular OS's.