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)

Implement the GNU extensions of mbsnrtowcs() and wcsnrtombs(). These are
convenient when the source string isn't null-terminated.

Implement the other conversion functions (mbstowcs(), mbsrtowcs(), wcstombs(),
wcsrtombs()) in terms of these new functions.

Allow encoding modules to override the default implementations of
mbsrtowcs() and wcsrtombs(). Provide a fast implementation for the
trivial "NONE" encoding.

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.

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().