support logind cgroup requests

Respond to the StartUnit and StartTransientUnit requests by logind
which are requests to create cgroups for the login session.
main: move ourselves into / cgroup at startup

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
shim: send the scope name and "done" in JobRemoved signal for Transient

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>