New module 'mbtowc'.

New module 'mbtowc'.

* lib/stdlib.in.h (mbtowc): New declaration.
* lib/mbtowc.c: New file.
* lib/mbtowc-impl.h: New file, from libutf8 with modifications
* m4/mbtowc.m4: New file.
* m4/stdlib_h.m4 (gl_STDLIB_H_DEFAULTS): Initialize GNULIB_MBTOWC,
REPLACE_MBTOWC.
* modules/stdlib (Makefile.am): Substitute GNULIB_MBTOWC,
REPLACE_MBTOWC.
* modules/mbtowc: New file.
* tests/test-stdlib-c++.cc: Test signature of mbtowc.
* doc/posix-functions/mbtowc.texi: Mention the new module.
* modules/btowc (Depends-on): Add mbtowc.