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.