systemctl: show main and control PID explicitly in cgroup-show

systemctl: show main and control PID explicitly in cgroup-show

In some cases the main/control PID of a service can be outside of the
services cgroups (for example, if logind readjusts the processes'
cgroup). In order to clarify this for the user show the main/control PID
in the cgroup tree nonetheless, but mark them specially.