* mingwex/wcrtomb.c: New file.
* mingwex/btowc.c: New file.
* mingwex/wctob.c: New file.
* mingwex/mb_wc_common.h: New file.
* mingwex/Makefile.in (DISTFILES): Add new files.
(Q8_OBJS): Add new objects.
* include/wchar.h: Adjust comment about mbrtowc() and related
funcions. Add __restrict__ to pointer params in prototypes.
(wmemset. wmemchr, wmemcpy, wmemmove, wcstoll, wcstoull): Remove
arg names from protototypes.
