New module 'wcslen'.

New module 'wcslen'.

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