Univention Corporate Server 4.1 erratum 309 (bugfix)

This update addresses the following issue(s):
* Locking in the sysvol sync script has been improved.
* In case Samba 4 is installed only on a DC backup the re-join fails. The
  join script now checks if this system is configured as S4 connector
  system and re-initialized the system. In this case all other Samba 4 DCs
  need to be re-joined.
* The idmap database is now removed while re-joining the system. This
  reduces the re-join time.
* The following UCR variables have been added to configure the winbind 
  settings: samba/winbind/nested/groups, samba/winbind/rpc/only,
  samba/winbind/max/clients, samba/winbind/enum/users and 
  samba/winbind/enum/groups.
* It is possible that Samba 4 joins against another DC and not against
  the master. This could led to different problems. The join script
  now tries to join against the S4 Connector system first.
* The group DC Backup Hosts is now member of the group Enterprise
  Domain Controllers. This is needed for the sysvol replication in
  case Samba 4 is not installed on the DC master.