(PATH_MAX): Define it if not yet defined.

(PATH_MAX): Define it if not yet defined.
(PATH_SEPARATOR, USE_GEN_PATH, GEN_PATH, GEN_PATH_FROM_MT): New
macros.
(get_database_filename): Use GEN_PATH.
(get_dir_info): Don't include the last '/' in directory name.
(register_database): New function.
(update_database_list): Use register_database.  Search all
directories in mdatabase__dir_list for wildcard databases.
(mdatabase__init): Check "~/.m17n.d".