(<wchar.h>): Do not include; no longer needed, since we no longer use wcrtomb.

(<wchar.h>): Do not include; no longer needed, since we no longer use wcrtomb.
(<limits.h>): Include if HAVE_LIMITS_H.
(MB_LEN_MAX): Define if limits.h doesn't.
(convert_unicode_char): New function, with the conversion part of the
old print_unicode_char.  Use wctomb, not wcrtomb.  Depend on
HAVE_WCTOMB, not HAVE_WCRTOMB.
(print_unicode_char): Use convert_unicode_char to do the conversion.