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

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

libc: Eliminate 13 relative relocations in wctype().

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)

Don't allocate the constant array "props" on the stack in wctype.

PR: 74743
Submitted by: knut st. osmundsen
Approved by: rwatson (mentor)
MFC after: 1 month

Recognize the "rune" character class in wctype().

Change wctype_t to an unsigned type to avoid warnings.

Add ISO C90 Amd. 1 wctype(3) and iswctype(3) functions.