core: when deserializing a unit, fully restore its cgroup state
The state of a unit was not fully restored, especially the
"cgroup_realized_mask/cgroup_enabled_mask" fields were missing.
This could be seen with the following sequence:
$ systemctl show -p TasksCurrent sshd
TasksCurrent=1
$ systemctl daemon-reload
$ systemctl show -p TasksCurrent sshd
TasksCurrent=
18446744073709551615
This was also visible with the "status" command: "Tasks: " row wasn't
showed in status of a service after a "daemon-reload" command.