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)

Reduce overhead by calling internal versions of the multibyte conversion
functions directly wherever possible.

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

Implement btowc() in terms of mbrtowc() instead of sgetrune(), and
wctob() in terms of wcrtomb() instead of sputrune(). There should be
no functional differences, but there may be a small performance hit
because we make an extra function call.

The aim here is to have as few functions as possible calling
s{get,put}rune() to make it easier to remove them in the future.

Add ISO C90 Amd. 1 btowc(3) and wctob(3) functions.