C.f. 287419c119ef961db487a281162ab037eba70c61: 'systemctl exit 42' can be
used to set an exit value and pulls in exit.target, which pulls in systemd-exit.service,
which calls org.fdo.Manager.Exit, which calls method_exit(), which sets the objective
to MANAGER_EXIT. Allow the same to happen through SuccessAction=exit.
v2: update for 'exit' and 'exit-force'
We would accept e.g. FailureAction=reboot-force in user units and then do an
exit in the user manager. Let's be stricter, and define "exit"/"exit-force" as
the only supported actions in user units.
v2:
- rename 'exit' to 'exit-force' and add new 'exit'
- add test for the parsing function
