(Fsave_charset_mapping_table): Use `open_chise_data_source_maybe'; use

(Fsave_charset_mapping_table): Use `open_chise_data_source_maybe'; use
`chise_ds_open_decoding_table' instead of `chise_open_decoding_table';
refer `default_chise_data_source'; use `chise_dt_close' instead of
`chise_close_decoding_table'.
(load_char_decoding_entry_maybe): Likewise.