ircbot: refactor and clean up botclass dir handling

ircbot: refactor and clean up botclass dir handling

Use File.join across the board, and refactor some botclass directory
handling. Most important changes:
 * failure to create the registry and safe_save directory is now fatal;
 * failure to create the local plugin directory prevents it from being
   added to the plugin path (with a warning);
 * botclass directory update from templates is now a standalone routine
   called during init, making it possible to use it in other cases too.