mbiter, mbfile, mbmemcasecoll: Improve handling of mbrtoc32 result.
* lib/mbiter.h (mbi_avail): If cur.ptr has reached the limit but
in_shift is true, call mbiter_multi_next.
(mbiter_multi_next): Set in_shift to false after an incomplete multibyte
character.
* lib/mbfile.h (mbfile_multi_getc): Pass the input bytes incrementally
into mbrtoc32. When mbf->state is not in the initial state, call
mbrtoc32 again.
* lib/mbmemcasecoll.c (apply_c32tolower): When the state is not in the
initial state, call mbrtoc32 again.