cgroups: Add systemd implementation of cgroups

This implements cgroup.Apply() using the systemd apis.
We create a transient unit called "docker-$id.scope" that contains
the container processes. We also have a way to set unit specific
properties, currently only defining the Slice to put the
scope in.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)

cgroups/apply_nosystemd.go

@@ -0,0 +1,15 @@
+// +build !linux
+
+package cgroups
+
+import (
+	"fmt"
+)
+
+func useSystemd() bool {
+	return false
+}
+
+func systemdApply(c *Cgroup, pid int) (ActiveCgroup, error) {
+	return nil, fmt.Errorf("Systemd not supported")
+}