New module 'wcsncpy'.

New module 'wcsncpy'.

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