core: when deserializing a unit, fully restore its cgroup state

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.