Univention Corporate Server 4.0 erratum 53 (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.
- univention-add-app now has an option to list all available App IDs using
  univention-add-app --list
- Delay requested reboot after all updates have been performed.
- Adapted the link to the new errata overview page inside the updater module.
- Open the Software Update UMC module upon UMC startup during release updates
  and show a information dialog about the current update process.
- 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.
- Adjust the release pre-update script to rename blocking old samba tdb files