(decode_builtin_char): Use `decode_builtin_char' instead of

(decode_builtin_char): Use `decode_builtin_char' instead of
`DECODE_CHAR' for mother; don't use special code for chinese-big5 to
use code space of chinese-big5-1 and chinese-big5-2.
(complex_vars_of_mule_charset): Use `MIN_CHAR_BIG5_CDP' and
`MAX_CHAR_BIG5_CDP' for chinese-big5.