Univention Corporate Server 4.4 erratum 50 (bugfix)

This update addresses the following issues:
* Searching for IP addresses of DNS host records is now possible.
  The Zone Time-to-Live and Mail Exchanger properties of DNS objects
  aren't shown in the search field list anymore in UMC.
* A crash of UDM is prevented which was caused by an extended attribute
  that overwrites a not existing layout group.
* The object class univentionPolicyReference is now removed from objects
  when the last policy is dereferenced.
* The search filter dnsAlias in the computer modules has been repaired.