Univention Corporate Server 3.2 erratum 253 (bugfix)

This update addresses the following issues:
- When using the command line tool 'univention-upgrade' all components
  blocking an update are now displayed.
- Detect broken HTTP proxies like DansGuardian, which block downloading the
  Packages files and updater scripts while still signalling success.
- The hints and warning messages before or during the execution of a UCS
  update have been clarified.
- Extended the message shown in the UMC module if an app is blocking 
  the update.
- List all blocking apps (or other components) in the UMC module.