(mark_char_table): Mark `ct->db_file' and `ct->db' in UTF-2000.

(char_table_description): Add description for `db_file' and `db' in
UTF-2000.
(Fmake_char_table): Initialize `ct->db_file' and `ct->db' in UTF-2000.
(Fcopy_char_table): Copy `ct->db_file' and `ct->db' in UTF-2000.
(Fsave_char_attribute_table): Use `ct->db_file' and `ct->db'.
(Fclose_char_attribute_table): New function.
(Freset_char_attribute_table): Reset `ct->db_file' and `ct->db'.
(load_char_attribute_maybe): Change interface; use `cit->db_file' and
`cit->db'.
(Fload_char_attribute_table): Use `ct->db_file' and `ct->db'.
(syms_of_chartab): Add new builtin function
`Fclose_char_attribute_table'.