The two ways of disabling session management have the same impact on the
session being saved, but there is one behavioural side-effect that
turned out to be less ideal.

By disabling completely we don't follow the session manager telling the
application to quit. That's not something needed with the systemd boot,
but for the legacy boot effectively we were just closing applications by
ripping the X connection away from under them.

Some applications are bad at handling this and this led to a bunch of
crashes or dangling processes at logout.

This reverts commit 9be7dedb.

CCBUG: 424408