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>

Cleanup: correct variable name in mbrtowc() signature.

Switch wchar from glibc to our own implementations.

Activate our new multibyte implementation.

* add implementations for the following multibyte-related
functions:
btwoc()
mblen()
mbrlen()
mbrtowc()
mbsinit()
mbtowc()
wcrtomb()
wcswidth()
wctob()
wctomb()
* the implementation of the above function live in a symbol
named __<name>, the above symbol names are defined as a weak
alias to the internal ones - TODO: we need to make sure to
only invoked the internal functions (i.e. prepended with __)
in order to avoid problems with symbol preemption.
* deactivate the limited mb implementation we provided before,
as well as respective stuff from glibc

Add private headers for internal versions of wchar/multibye-functions.

git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@43090 a95241bf-73f2-0310-859d-f6bbb57e9c96

Cleanup: correct variable name in mbrtowc() signature.

Switch wchar from glibc to our own implementations.

Activate our new multibyte implementation.

* add implementations for the following multibyte-related
functions:
btwoc()
mblen()
mbrlen()
mbrtowc()
mbsinit()
mbtowc()
wcrtomb()
wcswidth()
wctob()
wctomb()
* the implementation of the above function live in a symbol
named __<name>, the above symbol names are defined as a weak
alias to the internal ones - TODO: we need to make sure to
only invoked the internal functions (i.e. prepended with __)
in order to avoid problems with symbol preemption.
* deactivate the limited mb implementation we provided before,
as well as respective stuff from glibc

Add private headers for internal versions of wchar/multibye-functions.

git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@43090 a95241bf-73f2-0310-859d-f6bbb57e9c96