New module 'wcsdup'.

New module 'wcsdup'.

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