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)

Provide trivial macro implementations of getwc(), getwchar(), putwc() and
putwchar() to reduce function call overhead.

Remove masking macros for getwc(), putwc(), putwchar() and getwchar().
Although there was nothing wrong with getwc() and putwc(), getwchar()
and putwchar() assumed that <stdio.h> had been included before <wchar.h>,
which is not allowed by the standard.

Basic support for wide character I/O: getwc(), fgetwc(), getwchar(),
putwc(), fputwc(), putwchar(), ungetwc(), fwide().