Univention Corporate Server 4.3 erratum 196 (bugfix)

This update addresses the following issues:
* Backport for working around an issue in APR's trunc API. This is a
  prerequisite for the SHA1/shattered fixes.
* Backport to prevent the possibility of rep-sharing between a directory rep
  and a file/prop rep.
* Backport to reject commits which would introduce hash collisions with
  existing data, thus addressing the SHA1/shattered issue.