Don't include <limits.h>.

Don't include <limits.h>.
Include <stdlib.h> and <string.h> unconditionally.
(iswcntrl, mbsinit, ISCNTRL): New macros.
(mbsnwidth): Use K&R style function declarations.
Don't bother checking for MB_LEN_MAX == 1, since the compiler
can optimize it when MB_CUR_MAX == 1.
The width of control characters is zero, not 1.