New module 'wcsncat'.

New module 'wcsncat'.

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