Univention Corporate Server 4.2 erratum 121 (bugfix)

This update addresses the following issue(s):
* LDAP-Attributes which are required by their schema are now marked as
  required in UMC.
* Some performance optimizations during modification of user and group
  objects has been done.
* The next free sambaRID is used when only a S4 Connector with IPv6 is part
  of the domain.
* The error handling during creation of network objects with an invalid
  netmask has been improved.
* A regression in univention.admin.objects.get() has been fixed which caused
  that the Univention Corporate Client UMC configuration was not possible
  anymore.