Fix cgroup checks for containers

So far, from what I've noticed, is that you can perfectly fine run
processes with a cgroup line of 0::/.

I've been playing around with a non-systemd distro to see how they
behave. I picked Alpine, which uses OpenRC. When you open a shell,
you are actually in the 0::/ cgroup. But, as soon as you launch it
on boot using the init system, it assigns you a cgroups service is
running.

As time is limited, I assume that most init systems will assign a
cgroup with prefix if you run a service when cgroups are enabled.
Therefore, we now assume that we have an invalid configuration if the cgroup prefix is empty.

Author: Kevinjil

judge/create_cgroups.in

@@ -34,8 +34,8 @@ You can try using cgroup V1 by adding systemd.unified_cgroup_hierarchy=0 to the
     if ! echo "+cpuset" >> /sys/fs/cgroup/cgroup.subtree_control; then
         cgroup_error_and_usage "Error: Cannot add +cpuset to cgroup.subtree_control; check kernel params."
     fi
-    if ! grep -q "slice" /proc/self/cgroup; then
-        cgroup_error_and_usage "Error: Cgroups not configured properly, missing systemd slice under /proc/self/cgroup. If running under docker, make sure to set cgroupns=host."
+    if grep -q ":/$" /proc/self/cgroup; then
+        cgroup_error_and_usage "Error: Cgroups not configured properly, missing cgroup hierarchy prefix under /proc/self/cgroup. If running in a container, make sure to set cgroupns=host."
     fi
 
 else # Trying cgroup V1: