idgames_db_dump: shortened file param name for $db->add_file
Database: added get_file_by_[id|path] methods and POD

- Shortened file param name for add_file()
wadindex: issue #87: query for File objects by path/filename

- Use 'get_file_by_path' to retrieve the file ID for a file object
  from the database
Database: issue #87: implement get_file_by_path()