core: define "exit" and "exit-force" actions for user units and only accept that

core: define "exit" and "exit-force" actions for user units and only accept that

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