Univention Corporate Server 4.2 erratum 616 (extended maintenance)

This update addresses the following issues:
* In the cron package the postinst maintainer script allows for
  group-crontab-to-root privilege escalation via symlink attacks against
  unsafe usage of the chown and chmod programs. (CVE-2017-9525)
* calloc() return value resulting in remote DoS (CVE-2019-9704)
* DoS(memory consumption) via a large crontab file (CVE-2019-9705)
* Use-after-free resulting in DoS (CVE-2019-9706)