New module 'wcpncpy'.

New module 'wcpncpy'.

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