After setting "apt-mark unhold xserver-xorg-core" the output of DU is:
[08:00:29] root@thinkpad-s540:/home/jorg# apt-get dist-upgrade -dV
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
The following packages will be REMOVED:
xserver-xorg-video-ati (7.5.0-1+b1)
xserver-xorg-video-intel (2.21.15-2+b2)
xserver-xorg-video-mach64 (6.9.4-2)
xserver-xorg-video-r128 (6.9.2-1+b2)
The following packages will be upgraded:
xserver-xorg-core (1.16.4-1 => 1.17.1-2)
xserver-xorg-video-fbdev (0.4.4-1+b2 => 0.4.4-1+b3)
xserver-xorg-video-mga (1.6.4-1 => 1.6.4-1+b1)
xserver-xorg-video-nouveau (1.0.11-1 => 1.0.11-1+b1)
xserver-xorg-video-radeon (7.5.0-1 => 7.5.0-1+b1)
xserver-xorg-video-vesa (2.3.3-1+b3 => 2.3.3-1+b4)
xserver-xorg-video-vmware (13.1.0-1 => 13.1.0-1+b1)
7 upgraded, 0 newly installed, 4 to remove and 0 not upgraded.
Need to get 4,127 kB of archives.
After this operation, 3,740 kB disk space will be freed.
Do you want to continue? [Y/n] n
Abort.
I decided to abort the DU, to "apt-mark hold xserver-xorg-core" again (because it looks to risky, at least for me!) and to wait a few more days.
Greetings,
Jörg