Break down groups into subsystems

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)

cgroups/fs/cpu.go

@@ -0,0 +1,23 @@
+package fs
+
+import (
+	"strconv"
+)
+
+type cpuGroup struct {
+}
+
+func (s *cpuGroup) Set(d *data) error {
+	// We always want to join the cpu group, to allow fair cpu scheduling
+	// on a container basis
+	dir, err := d.join("cpu")
+	if err != nil {
+		return err
+	}
+	if d.c.CpuShares != 0 {
+		if err := writeFile(dir, "cpu.shares", strconv.FormatInt(d.c.CpuShares, 10)); err != nil {
+			return err
+		}
+	}
+	return nil
+}