(Vcharset_jis_x0208): New variable in UTF-2000.

(Vcharset_jis_x0208): New variable in UTF-2000.
(Qjis_x0208): Likewise.
(decode_builtin_char): Prefer XCHARSET_MAX_CODE(charset) than
XCHARSET_FINAL(charset).
(charset_code_point): Modify condition for final-byte based
builtin-chars.
(syms_of_mule_charset): Add new symbol `=jis-x0208' in UTF-2000.
(complex_vars_of_mule_charset): Add new coded-charset `=jis-x0208' in
UTF-2000; specify `=jis-x0208' as the mother of
`japanese-jisx0208-1978', `japanese-jisx0208' and
`japanese-jisx0208-1990' in UTF-2000.