bpo-30647: Check nl_langinfo(CODESET) in locale coercion (GH-2374)

- On some versions of FreeBSD, setting the "UTF-8" locale
  succeeds, but a subsequent "nl_langinfo(CODESET)" fails
- adding a check for this in the coercion logic means that
  coercion will happen on systems where this check succeeds,
  and will be skipped otherwise
- that way CPython should automatically adapt to changes in
  platform behaviour, rather than needing a new release to
  enable coercion at build time
- this also allows UTF-8 to be re-enabled as a coercion
  target, restoring the locale coercion behaviour on Mac OS X