Fix lxcfs cpuinfo

[bduffany]

Fix lxcfs cpuinfo not respecting cpuset when executor.child_cgroups_enabled is set.

On initialization, lxcfs looks up the current cgroup for pid 1 (in the current pid namespace) and makes an assumption that this cgroup's controllers will be the same controllers that are enabled for all child cgroups: https://github.com/lxc/lxcfs/blob/42b57f265a31261a0b69dc1f040cbcfe58a9c6a1/src/cgroups/cgfsng.c#L1036-L1054

So to make lxcfs happy, we need to enable cgroup controllers in the executor cgroup before starting lxcfs.

The fact that we aren't explicitly enabling cgroup controllers in the child cgroups made me wonder whether cgroups were working at all. After some experimentation, it seems that these cgroup controllers are only getting enabled because crun automatically enables them for us (based on the contents of buildbuddy.executor/cgroup.controllers before and after running a container). We can't rely on this automatic setup for lxcfs though, because at the time when we start lxcfs, we haven't yet invoked crun for the first time, so it hasn't set up the controllers for us.

[bduffany]

This change is not strictly needed but I realized this function name and documentation are misleading. ../cgroup.subtree_control returns the set of cgroup controllers that are enabled for children of the parent cgroup, not the controllers enabled for the parent cgroup itself. So it should really be called "EnabledControllers" not "ParentEnabledControllers." Also it's probably clearer to read ./cgroup.controllers rather than ../cgroup.subtree_control, even though (iiuc) they should always return the same set of controllers.