9406: Add -cgroup-parent-subsystem argument.

9406: Add -cgroup-parent-subsystem argument.

This accommodates setups where the scheduling system (e.g., SLURM)
uses cgroups to impose resource limits on crunch-run itself, and those
limits are meant to include resources used by the container itself.

Normally, when docker creates a container, it creates new cgroups
inside the "docker" cgroup, which circumvents any resource limits set
on crunch-run's own cgroup.

If a subsystem is specified (e.g., -cgroup-parent-subsystem=memory),
crunch-run will look in /proc/self/cgroup to find the current cgroup
for the given subsystem, and instruct docker to use that cgroup as the
parent of the new container's cgroup instead.