Univention Corporate Server 4.2 erratum 601 (extended maintenance)

This update addresses the following issues:
* Backport recent stretch update (+deb9u3) of FreeRDP v1.1 to Debian jessie
  LTS.
* Integer truncation leading to heap-based buffer overflow in
  update_read_bitmap_update() function (CVE-2018-8786)
* Integer overflow leading to heap-based buffer overflow in
  gdi_Bitmap_Decompress() function (CVE-2018-8787)
* Out-of-bounds write in nsc_rle_decode() function (CVE-2018-8788)
* Several out-of-bounds reads in NTLM authentication module resulting in a
  denial of service (CVE-2018-8789)