login: fix re-use of users

If the last reference to a user is released, we queue stop-jobs for the
user-service and slice. Only once those are finished, we drop the
user-object. However, if a new session is opened before the user object is
fully dropped, we currently incorrectly re-use the object. This has the
effect, that we get stale sessions without a valid "systemd --user"
instance.

Fix this by properly allowing user_start() to be called, even if
user->stopping is true.