Univention Corporate Server 4.2 erratum 287 (bugfix)

This update addresses the following issues:
* Kerberos authentication failed due to expired keys when the domain wide
  Samba maxPwdAge setting was is too large. UDM and UMC now restrict the
  values that can be set for sambaMaxPwdAge and sambaMinPwdAge.
* It is now prevented to set mail addresses without local part or without
  domain part.