New module 'towctrans'.

New module 'towctrans'.

* modules/towctrans: New file.
* lib/wctype.in.h (towctrans): New declaration.
* lib/towctrans.c: New file.
* lib/towctrans-impl.h: New file.
* m4/towctrans.m4: New file.
* m4/wctype_h.m4 (gl_WCTYPE_H): Test whether towctrans is declared.
(gl_WCTYPE_H_DEFAULTS): Initialize GNULIB_TOWCTRANS.
* modules/wctype-h (Makefile.am): Substitute GNULIB_TOWCTRANS.
* tests/test-wctype-h-c++.cc: Test the declaration of towctrans.
* doc/posix-functions/towctrans.texi: Mention the new module.