(decode_defined_char): New function; search mother.

(decode_defined_char): New function; search mother.
(decode_builtin_char): Don't search mother if
XCHARSET_MAX_CODE(charset) == 0.
(charset_code_point): Search mother if XCHARSET_MAX_CODE(charset) == 0
even if code >= XCHARSET_MAX_CODE(charset).
(Fdecode_char): Use `decode_defined_char' instead of
`DECODE_DEFINED_CHAR'.
(complex_vars_of_mule_charset): Specify `Vcharset_ucs' as the mother
of `Vcharset_ucs_jis'.