Reset persistent mbstates when rune locale encoding changes.

This was shown to be a problem by side effect of now-enabled test case,
which was going through C, en_US.UTF-8, ja_JP.SJIS, and ja_JP.eucJP,
and failing eventually as data in mbrtowc's mbstate, that was
perfectly correct for en_US.UTF-8 was treated as incorrect for
ja_JP.SJIS, failing the entire test case.

This makes the persistent mbstates to be per ctype-component,
and not per-locale so we could easily reset the mbstates when
only LC_CTYPE is changed.

Reviewed by: bapt, pfg
Approved by: kib (mentor, implicit)
Differential Revision: https://reviews.freebsd.org/D17796

Fix and re-enable UTF-8 tests.

Mechanically replace #if defined(__FreeBSD__) and #if defined(__NetBSD__) with
their #ifdef equivalents for everything changed in contrib/netbsd-tests. There
are some items from the vendor tree that use #if defined(__FreeBSD__) or
#if defined(__NetBSD__) which are being left alone

Requested by: bde, rpaulo
Sponsored by: EMC / Isilon Storage Division

Do initial port of contrib/netbsd-tests/lib/libc/locale

t_io:
- Expect failures potentially related to implementation-specific knowledge of
the zh_TW.Big5 locale [*]

t_mbrtowc:
- Handle unknown locales more gracefully (do not test if the locale doesn't
exist)
- Expect failure with mbrtowc_internal dealing with Japanese locales
(potentially related to implementation detail knowledge of the ja_* locales) [*].

t_mbstowcs, t_mbtowc, t_wctomb:
- Handle unknown locales more gracefully (do not test if the locale doesn't
exist)

t_wcstod:
- Treat FreeBSD like NetBSD and Linux in the XXX: FIXME section

[*] More investigation is required to determine the root cause of the failures

Submitted by: pho
Sponsored by: EMC / Isilon Storage Division