Univention Corporate Server 4.3 erratum 355 (bugfix)

This update addresses the following issue(s):
* The UDM module "settings/data" was added. It can be used to store
  arbitrary data in LDAP.
* The username 'admin' is now allowed
* Added a new hook base class: "AttributeHook". This can be used to
  implement a mapping between LDAP and UDM for extended attributes.
* Keep the choices for the modules in extended attributes updated.
* System accounts are not counted if the license allows unlimited users.