Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Implement xlocale APIs from Darwin, mainly for use by libc++. This adds a
load of _l suffixed versions of various standard library functions that use
the global locale, making them take an explicit locale parameter. Also
adds support for per-thread locales. This work was funded by the FreeBSD
Foundation.

Please test any code you have that uses the C standard locale functions!

Reviewed by: das (gdtoa changes)
Approved by: dim (mentor)

Prepare to handle state-dependent encodings. This mainly involves not
taking shortcuts when it comes to storing and passing around conversion
states.

Pass NULL instead of a pointer to a zeroed mbstate_t object.

Add a placeholder implementation of wcscoll() and wcsxfrm() which gives
locale-sensitive collation only in single-byte locales, and just does
binary comparison for the others with extended character sets.