Bug#834789: this upgrade knocks one out of X-windows

[積丹尼 Dan Jacobson]

Package: systemd
Version: 231-4

Doing

    Setting up systemd (231-4) ...
    addgroup: The group `systemd-journal' already exists as a system group. Exiting.

while nodm is running knocks the user into a black screen.

To recover: CTRL ALT F1, then login, then do chvt 7... perhaps some
users won't figure out how to recover...

Seen on each of my several computers.