New module 'wmemset'.

New module 'wmemset'.

* modules/wmemset: New file.
* lib/wchar.in.h (wmemset): New declaration.
* lib/wmemset.c: New file.
* lib/wmemset-impl.h: New file, from libutf8 with modifications.
* m4/wmemset.m4: New file.
* m4/wchar_h.m4 (gl_WCHAR_H): Test whether wmemset is declared.
(gl_WCHAR_H_DEFAULTS): Initialize GNULIB_WMEMSET, HAVE_WMEMSET.
* modules/wchar (Makefile.am): Substitute GNULIB_WMEMSET, HAVE_WMEMSET.
* tests/test-wchar-c++.cc: Test the declaration of wmemset.
* doc/posix-functions/wmemset.texi: Mention the new module.