Univention Corporate Server 4.0 erratum 76 (bugfix)

* UMC specific apache configuration has been put into a LocationMatch
  directive to not disturb other services usiong apache.
* Don't let firefox ask for resending POST form data when logging out
  of UMC.
* Fix an endless login animation if the login fails due to not running
  UMC server.
* Fix the error message which is shown when no network connection is
  possible on client side.
* Fix visibility of error messages when a login dialog is opened.
* Add more detailed error messages if no connection to UMC server
  components is possible.