[$] A milestone for control groups

Changes to core-kernel subsystems take time but, even so, one can onlyimagine that Tejun Heo never expected the process of fixing thecontrol-group interface to take more than five years. Disagreements overthe design of the new control-group interface have delayed its adoption;even though most of the code has been in the kernel for some time, not allcontrollers work with it. It would now appear, however, that agreement hasbeen reached on an important final piece, which is currently on track to bemerged for the 4.14 development cycle.