Univention Corporate Server 4.1 erratum 215 (bugfix)

This update addresses the following issues:
* IPv6 support was added to "get_notifier_id.py".
* The help messages for "univention-directory-listener-ctrl" were improved.
  Also two new subcommand 'modules' and 'status' have been added.
* Some string and integer comparison bugs have been fixed.
* The Listener did not drop root privileges in all error cases.
* Some old and no longer needed code has been removed to reduce the memory
  footprint.
* Some debug messages were cleaned up.
* Some data structures are no longer allocated dynamically but put onto the
  stack to make the Listener more robust against memory allocation problems.
* A bug in handling the notifier ID has been fixed: If the Listener was
  restarted multiple times, the last processed transaction ID could be lost.
  This led to all transactions being skipped which happened in between.
* The locking has been improved to prevent multiple instances of the Listener
  running at the same time.
* The transaction log is now only written on systems where the Notifier is
  installed as well to prevent the hard disk from filling up.