Univention Corporate Server 4.0 erratum 381 (bugfix)

This update addresses the following issues:
* The ACL check in the sysvol sync script is now limited
  to the Policies directory.
* There are two new UCR variables samba4/sysvol/sync/from_upstream
  and samba4/sysvol/sync/from_downstream which can be used to deactivate
  copying files from other DCs.
* The sysvol-sync script now checks if any changes need to be
  synchronized at all and it uses file locking to coordinate
  concurrent read and write processes.