* lib/uchar.in.h (c32srtombs): New declaration.
* lib/wcsrtombs-impl.h: Parameterize: Use macros FUNC, SCHAR_T,
INTERNAL_STATE, WCRTOMB.
* lib/wcsrtombs.c (FUNC, SCHAR_T, INTERNAL_STATE, WCRTOMB): New macros.
* lib/c32srtombs.c: New file.
* lib/c32srtombs-state.c: New file, based on lib/wcsrtombs-state.c.
* m4/uchar.m4 (gl_UCHAR_H_DEFAULTS): Initialize GNULIB_C32SRTOMBS.
* modules/uchar (Makefile.am): Substitute GNULIB_C32SRTOMBS.
* modules/c32srtombs: New file.
* tests/test-uchar-c++.cc: Test the signature of c32srtombs.
* doc/posix-functions/wcsrtombs.texi: Mention the new module.