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>

strfmon.c: Fix -Wpointer-compare

Fix comparison between pointer and zero character constant.
Pointed out by gcc7.

See also: https://github.com/freebsd/freebsd/commit/3d4c1b73b9036a1d5455a4a63eba6e0d20be9350

Change-Id: I917ceefce69e269fbaa12a3ae60f0702f5c5d4a7
Reviewed-on: https://review.haiku-os.org/782
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>

Introduce __set_errno() throughout libroot.

* add errno_private.h, which defines the __set_errno() macro with
and without tracing
* instead of setting errno manually, all libroot's code now invokes
__set_errno(), which makes it much easier to trace changes to errno
* redirect glibc's use of __set_errno() to our own version

* reintegrated posix-locale

git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@37725 a95241bf-73f2-0310-859d-f6bbb57e9c96

Introduce __set_errno() throughout libroot.

* add errno_private.h, which defines the __set_errno() macro with
and without tracing
* instead of setting errno manually, all libroot's code now invokes
__set_errno(), which makes it much easier to trace changes to errno
* redirect glibc's use of __set_errno() to our own version

* reintegrated posix-locale

git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@37725 a95241bf-73f2-0310-859d-f6bbb57e9c96