Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

MFC the xlocale implementation.

Merged revisions: 227487,227753,227807,227818,227999,228002,228875,230156,231673,232498,232601,232620,232626,232926-232927,232929,232931,232935,233173,233600,234573,234578,235239

This currently defines __NO_TLS on ARM (unlike head), because the required
support function has not been MFC'd.

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

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.

When called with s == NULL, behave as if wc == L'\0' as required by the
standard.

Use an internal buffer for the result when the first argument is NULL.

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().