New POSIX xlocale implementation written from scratch.
Complete in the sense that all POSIX *locale(3) and *_l(3) functions
are included, but in OpenBSD, we of course only really care about
LC_CTYPE and we only support ASCII and UTF-8.

With important help from kettenis@, guenther@, and jca@.
Repeated testing in ports bulk builds by naddy@.
Additional testing by jca@, sebastia@, dcoppa@, and others.
OK kettenis@ dcoppa@, and guenther@ on an earlier version.
Riding guenther@'s libc/librthread major bump.

Delete some abstraction that we don't and won't need.
Declare functions rather than generating declarations with macros.
Just call functions rather than mainting function pointer tables.
Purge unused arguments. Simplify mbstate_t casting.
Garbage collect one empty and one unused function.

As a bonus, make mbsinit(3) work at all, it returned garbage
in the past due to a missing cast when passing mbstate_t.
Apart from that, no functional change.

No libc bump needed; only private functions are removed and
change prototype and only private structs change size.

OK stsp@ mpi@; deraadt@ likes the general direction.

Wrap <inttypes.h> and finish wrapping of <wchar.h> so that calls go direct
and the symbols not in the C standard are weak

Include stdint.h, not limits.h to get SIZE_MAX. OK guenther@

Remove excessive sys/cdefs.h inclusion
ok guenther millert kettenis

remove unused variable

ok matthew@

Add support for mbsnrtowcs() and wcsnrtombs() to libc.

Bulk build test by naddy.

Replace the single-byte placeholders for the multi-byte/wide-character
conversion interfaces of libc (mbrtowc(3) and friends) with new
implementations that internally call an API based on NetBSD's citrus.
This allows us to support locales with multi-byte character encodings.

Provide two implementations of the citrus-based API: one based on the old
single-byte placeholders for use with our existing single-byte character
locales (C, ISO8859-*, KOI8, CP1251, etc.), and one that provides support
for UTF-8 encoded characters (code based on FreeBSD's implementation).

Install the en_US.UTF-8 ctype locale support file, and allow the UTF-8
ctype locale to be enabled via setlocale(3) (export LC_CTYPE='en_US.UTF-8').

A lot of programs, especially from ports, will now start using UTF-8 if the
UTF-8 locale is enabled. Use at your own risk, and please report any breakage.
Note that ncurses-based programs cannot display UTF-8 right now, this is being
worked on.

To prevent install media growth, add vfprintf(3) and mbrtowc(3) to libstubs.
The mbrtowc stub was copied unchanged from its old single-byte placeholder.
vfprintf.c doesn't need to be copied, just put in .PATH (hint by fgsch@).

Testing by myself, naddy, sthen, nicm, espie, armani, Dmitrij D. Czarkoff.

ok matthieu espie millert sthen nicm deraadt