cgroup: run PID 1 in the root cgroup

This way cleaning up the cgroup tree on shutdown is a lot easier since
we are in the root dir. Also PID 1 was previously artificially placed in
system.slice, even though our rule actually was not to have processes in
slices. The root slice otoh is magic anyway, so having PID 1 in there
sounds less surprising.

Of course, this means that PID is scheduled against the three top-level
slices.