cgroup's Controller.move_tasks() API makes it difficult to implement CM #565

In order to implement a context manager that would nest correctly, we would need:

• cgroups.freeze() currently returns the non-frozen tasks. What would be needed is the opposite so that we can undo what have been done at this precise level.
• cgroups.freeze() takes an exclude parameter. It would need to take an include parameter to only affect the listed tasks