Univention Corporate Server 3.2 erratum 29 (bugfix)

This errata update fixes several issues:
* adjust the behaviour of ucs_registerLDAPExtension to let
  join scripts continue in case an LDAP extension could not
  be registered because a newer one is already active in the
  domain.
* Group name mapping has been added to univention-lib.
  The new function "custom_groupname" maps the default group names
  to the actual name.
* User name mapping has been added to univention-lib.
  The new function "custom_username" maps the default user names
  to the actual name.
* A new python lib (s4) has been added for Samba-related operations.