file-systems: Use cgroups v2.

cgroup v2 is the next generation of the control groups API.  This patch
replaces the cgroup v1 file systems with the unified cgroup v2 file
system.

cgroup v2 allows for things like containerd/podman to run rootless
containers and opens guix system up to running things like Kubernetes.

Thanks to Hilton Chain <hako@ultrarare.space> for suggesting the Docker
service change.

* gnu/system/file-systems.scm (%control-groups): Change to a single
"cgroup2" mount point.
* gnu/services/docker.scm (docker-shepherd-service): Trim 'requirement'
field accordingly.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>

gnu/services/docker.scm

@@ -116,12 +116,7 @@ loop-back communications.")
            (requirement '(containerd
                           dbus-system
                           elogind
-                          file-system-/sys/fs/cgroup/blkio
-                          file-system-/sys/fs/cgroup/cpu
-                          file-system-/sys/fs/cgroup/cpuset
-                          file-system-/sys/fs/cgroup/devices
-                          file-system-/sys/fs/cgroup/memory
-                          file-system-/sys/fs/cgroup/pids
+                          file-system-/sys/fs/cgroup
                           networking
                           udev))
            (start #~(make-forkexec-constructor

gnu/system/file-systems.scm

@@ -445,26 +445,13 @@ TARGET in the other system."
     (flags '(read-only bind-mount no-atime))))
 
 (define %control-groups
-  (let ((parent (file-system
-                  (device "cgroup")
-                  (mount-point "/sys/fs/cgroup")
-                  (type "tmpfs")
-                  (check? #f))))
-    (cons parent
-          (map (lambda (subsystem)
-                 (file-system
-                   (device "cgroup")
-                   (mount-point (string-append "/sys/fs/cgroup/" subsystem))
-                   (type "cgroup")
-                   (check? #f)
-                   (options subsystem)
-                   (create-mount-point? #t)
-
-                   ;; This must be mounted after, and unmounted before the
-                   ;; parent directory.
-                   (dependencies (list parent))))
-               '("cpuset" "cpu" "cpuacct" "memory" "devices" "freezer"
-                 "blkio" "perf_event" "pids")))))
+  ;; The cgroup2 file system.
+  (list (file-system
+          (device "none")
+	  (mount-point "/sys/fs/cgroup")
+	  (type "cgroup2")
+	  (check? #f)
+	  (create-mount-point? #f))))
 
 (define %elogind-file-systems
   ;; We don't use systemd, but these file systems are needed for elogind,