- Copy stable/10 (r259064) to releng/10.0 as part of the
10.0-RELEASE cycle.
- Update __FreeBSD_version [1]
- Set branch name to -RC1

[1] 10.0-CURRENT __FreeBSD_version value ended at '55', so
start releng/10.0 at '100' so the branch is started with
a value ending in zero.

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

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)

Move prototypes of various encoding-related functions into a new header
file to avoid extern'ing them all over the place.

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

Remove support for emulating mbrtowc() and wcrtomb() in terms of the
old rune interface now that it is no longer needed.

Allow mbrtowc() and wcrtomb() to be implemented directly, instead of
as wrappers around the deprecated 4.4BSD rune functions. This paves the
way for state-dependent encodings, which the rune API does not support.
- Add __emulated_sgetrune() and __emulated_sputrune(), which are
implementations of sgetrune() and sputrune() in terms of
mbrtowc() and wcrtomb().
- Rename the old rune-wrapper mbrtowc() and wcrtomb() functions to
__emulated_mbrtowc() and __emulated_wcrtomb().
- Add __mbrtowc and __wcrtomb function pointers, which point to the
current locale's conversion functions, or the __emulated versions.
- Implement mbrtowc() and wcrtomb() as calls to these function pointers.
- Make the "NONE" encoding implement mbrtowc() and wcrtomb() directly.

All of this emulation mess will be removed, together with rune support,
in FreeBSD 6.

Don't check whether the first byte of the buffer is a null byte when
the buffer has zero length (n == 0).

Style: One space between "restrict" qualifier and "*".

Implement the ISO C90 Amd.1 restartable wide and multibyte character
manipulation functions mbrlen(), mbrtowc(), mbsinit(), mbsrtowcs(),
wcrtomb(), wcsrtombs().