libs/posix: Implemented new locale functions

Implemented the missing POSIX functions in <locale.h>:
newlocale, duplocale, uselocale, and freelocale, and also
provided missing type definitions for <locale.h>.

Implemented missing POSIX locale-based function variants.

Modified LocaleBackend so that it could support thread-local
locales.

Some glibc-like locale-related variables supporting
ctype and printf family of functions have also been updated
to reflect the thread-local variables present in the latest
glibc sources.

As there have been some modifications to global symbols
in libroot, libroot_stubs.c has been regenerated.

Bug: #17168
Change-Id: Ibf296c58c47d42d1d1dfb2ce64042442f2679431
Reviewed-on: https://review.haiku-os.org/c/haiku/+/5351
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>

Fix double extern "C" pointed by clang.

Change-Id: Ie595dd99d01149868a4f60354ef89fa861cf856b
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2389
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

s/Haiku License/MIT License/g.

They are the same thing.

Cleanup: LocaleBackend.h is not a local header for wchar.

Fix POSIX-specific mbsrtowcs() and wcsrtombs().

* src was sometimes accessed incorrectly (needs double dereference)
* the source pointers may only be adjusted in case there is the
destination pointer is not NULL

Drop public symbols for mbsnrtowcs() and wcsnrtombs().

We only implement POSIX_VERSION 199009, so these needn't be exported.

Add implementation of mbsrtowcs() to our locale backend.

* add MultibyteStringToWchar() to ICU locale backend
* implement mbsrtowcs() and mbsnrtowcs() on top of
MultibyteStringToWchar()
* drop respective glibc files

Cleanup: LocaleBackend.h is not a local header for wchar.

Fix POSIX-specific mbsrtowcs() and wcsrtombs().

* src was sometimes accessed incorrectly (needs double dereference)
* the source pointers may only be adjusted in case there is the
destination pointer is not NULL

Drop public symbols for mbsnrtowcs() and wcsnrtombs().

We only implement POSIX_VERSION 199009, so these needn't be exported.

Add implementation of mbsrtowcs() to our locale backend.

* add MultibyteStringToWchar() to ICU locale backend
* implement mbsrtowcs() and mbsnrtowcs() on top of
MultibyteStringToWchar()
* drop respective glibc files