New module 'wcsnlen'.

New module 'wcsnlen'.

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