core: report root cgroup as "/" over the bus

core: report root cgroup as "/" over the bus

Internally, the root cgroup is stored as the empty string in
Unit.cgroup_path, and "no cgroup" as NULL. Unfortunately, D-Bus does not
know a NULL concept, hence when reporting the cgroup to clients we
should turn the root cgroup into "/", and leave the empty string for the
"no cgroup" case.

This should make sure that "systemctl status -- -.slice" works correctly
and shows the entire cgroup tree.