Check if any cgroup controller is currently being used as v1, and if so don't
use cgroup unified mode. This is only checked when there is no cgroup already
mounted, and is most useful when running in a container where the host is using
a legacy or hybrid cgroup, since we can't (fully) use unified cgroup, as the
host kernel has the cgroup controllers in use by v1.
This shouldn't be needed if the container manager has pre-mounted cgroups
appropriately based on the host system cgroup setup, but we can't rely
on that always being the case.
Dan Streetman
22eeada935