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