(default_chise_data_source): New variable when `CHISE' is defined.

(default_chise_data_source): New variable when `CHISE' is defined.
(Fcopy_char_table): `ct->ds' is deleted.
(open_chise_data_source_maybe): New function.
(Fclose_char_data_source): New function.
(char_table_open_db_maybe): Use `open_chise_data_source_maybe'; use
`chise_ds_open_feature_table' instead of `chise_open_feature_table'.
(char_table_close_db_maybe): Use `chise_ft_close' instead of
`chise_close_feature_table'; `cit->ds' is deleted.
(syms_of_chartab): Add new builtin function `close-char-data-source'
when `HAVE_CHISE_CLIENT' is defined.