libc: New functions c8rtomb(3) and mbrtoc8(3).

New in C23, for converting from UTF-8 to locale-dependent multibyte
sequences (c8rtomb) or vice versa (mbrtoc8), along with the new type
char8_t.

Conditional on either:
- _NETBSD_SOURCE
- _ISOC23_SOURCE
- __STDC_VERSION__ >= 202311L

(Riding the libc minor bump from this morning for the UTF-16/UTF-32
versions from C11.)

PR standards/58601: uchar.h C23 compliance: char8_t, mbrtoc8, c8rtomb

libc: New C11 functions mbrtoc16, mbrtoc32, c16rtomb, c32rtomb.

The mbrtoc16/32 functions read mulitbyte strings according to the
current locale into UTF-16/32 code unit sequences; the c16/32rtomb
functions write UTF-16/32 code unit sequences into multibyte strings
according to the current locale. The `r' means restartable: they
work incrementally and pick up where they left off.

NOTE: This bumps the libc minor version, since it adds new symbols.

PR lib/52374: <uchar.h> missing

uchar.h: New header file for C11 (and C++11) compliance.

Implementation of the new functions mbrtoc16, c16rtomb, mbrtoc32, and
c32rtomb to come later. Updates for C23 to come later.

PR lib/52374: <uchar.h> missing

tests/lib/libc/locale/Makefile: Sort.

No functional change intended.

Preparation for PR lib/52374.

branches: 1.14.2;
Don't use fmtcheck for strfmon format strings. It does not work. Fix a broken
test.

branches: 1.13.10;
PR/54414: Valery Ushakov: add a test for wcsrtombs(3) doesn't update the
source argument on conversion error

branches: 1.12.4;
Add missing strfmon_l. Noticed by Bruno Haible. Add test case.

Add missing files from last commit:

Move Unicode <-> ku/ten mapping into the individual codec modules.
Mapping is based on existing iconv data for single-byte encodings,
and included for several, but not all, multibyte encodings.

branches: 1.10.2;
Add a simple collation test. This test is expected to fail on HEAD since
we do not yet have a working implementation of wcscoll.

branches: 1.9.2;
Add tests for btowc(3)/wctob(3) and enable compilation of the test for
digittoint(3).

The digittoint(3) test is skipped since we don't provide that function yet.

One of the test cases for btowc(3) is also skipped, since it tests conversion
to Unicode---whereas our wchar_t representation is locale-dependent.

Add test cases for sprintf/sscanf/strto{d,l} and the is* and isw* ctype functions, for single-byte encodings

Add simple test case for toupper/tolower

Add mbsnrtowcs and wcsnrtombs. Approved by core.

regression tests for wide char i/o. Currently there are failures.

branches: 1.4.6;
Add test cases for strcspn, strpbrk, strspn, wcscspn, wcspbrk and
wcsspn.

branches: 1.3.2;
Rename two test files to get functional scope (and avoid confusion
with ctype(3)). No functional change.

Fix build with stack smash protection enabled.

atf-ify the various locale tests