Univention Corporate Server 4.2 erratum 42 (bugfix)

This update addresses the following issues:
* Don't fail join if CNF-Objects are in serverReferenceBL
* Don't consider unreachable UCS@school Slave DCs for joining
* Support uppercase CN: attribute in sysvol-cleanup.py
* Adjust check_essential_samba4_dns_records.sh to change of behavior
  of the "host" command in UCS 4.2
* The UCR variables samba/client/min/protocol, samba/min/protocol
  and samba/client/max/protocol have been added.
  Please be aware that raising samba/min/protocol e.g. to SMB2 also
  requires raising samba/client/max/protocol to that value or higher.
* Make sysvol-sync.sh robust against varying stderr output from rsync.
* Update samba package and restart bind9 on upgrade to make bind9 load
  the updated dlz_bind9 module version, which contains improvements in
  the handling of DDNS updates.