The Synchronize() function is just too useful for clients, so that we
can make "systemd-run -v --user" actually useful. Hence let's make the
socket accessible without privs. Deny most method calls however, except
for the Synchronize() call.
If we have exit on idle, then operations such as "journalctl
--namespace=foo --rotate" should work even if the journal daemon is
currently not running.
(Note that we don't do activation by varlink for the main instance of
journald, I am not sure the deadlocks it might introduce are worth it)