[$] Cloning into a control group

The Linux control-group mechanism was designed to make it easy to assignprocesses to groups or move them around; it is a simple matter of writing aprocess ID to the appropriate cgroup.procs file in thecontrol-group filesystem hierarchy. That only works for processes thatactually exist, though. Adding the ability to place a new process into acontrol group at birth is the subject of thispatch set from Christian Brauner.