[$] Thread-level control with resource groups

The kernel's control-group mechanism allowsprocesses to be divided into groups for the purposes of tracking and resource control. Both the API andunderlying implementation of this mechanism have been going throughconsiderable change in recent years. As part of that change, the newer control-group API has lost theability to separately manage threads within a process, a loss that is not welcome in somequarters. Current work to replace that functionality is not finding anentirely warm reception either, though.