mbrtowc: Work around a Solaris 7 bug.

mbrtowc: Work around a Solaris 7 bug.

* m4/mbrtowc.m4 (gl_MBRTOWC_NULL_ARG1): New macro.
(gl_MBRTOWC_NULL_ARG2): Renamed from gl_MBRTOWC_NULL_ARG.
(gl_FUNC_MBRTOWC): Update. Define MBRTOWC_NULL_ARG2_BUG instead of
MBRTOWC_NULL_ARG_BUG. Invoke gl_MBRTOWC_NULL_ARG1 and define
MBRTOWC_NULL_ARG1_BUG.
* lib/mbrtowc.c (rpl_mbrtowc): Use MBRTOWC_NULL_ARG2_BUG instead of
MBRTOWC_NULL_ARG_BUG. Handle MBRTOWC_NULL_ARG1_BUG.
* tests/test-mbrtowc.c (main): Test support of a NULL first argument.
* doc/posix-functions/mbrtowc.texi: Mention the Solaris 7 bug.