(decode_defined_char): Don't refer external database if Qunbound or

(decode_defined_char): Don't refer external database if Qunbound or
Qnil are found.
(Fsave_charset_mapping_table): Don't define it if HAVE_CHISE_CLIENT is
not defined.
(Freset_charset_mapping_table): New function.
(load_char_decoding_entry_maybe): Store Qnil if a character is not
found in external database.
(syms_of_mule_charset): Don't define `save-charset-mapping-table' if
HAVE_CHISE_CLIENT is not defined; add new builtin function
`reset-charset-mapping-table' if HAVE_CHISE_CLIENT is defined.