(Qbig5_1): New variable in UTF-2000.

(Qbig5_1): New variable in UTF-2000.
(Qbig5_2): Likewise.
(decode_defined_char): Support CONVERSION_BIG5_1 and
CONVERSION_BIG5_2.
(decode_builtin_char): Likewise.
(charset_code_point): Likewise.
(Fmake_charset): Accept `big5-1' and `big5-2' as a value of
'conversion in UTF-2000.
(syms_of_mule_charset): Add new symbol `big5-1' and `big5-2'.
(complex_vars_of_mule_charset): Define `chinese-big5-1' and
`chinese-big5-2' as children of `chinese-big5'.