Refactor mounts into pkg to make changes easier

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

libcontainer/mount/msmoveroot.go

@@ -0,0 +1,19 @@
+// +build linux
+
+package mount
+
+import (
+	"fmt"
+	"github.com/dotcloud/docker/pkg/system"
+	"syscall"
+)
+
+func MsMoveRoot(rootfs string) error {
+	if err := system.Mount(rootfs, "/", "", syscall.MS_MOVE, ""); err != nil {
+		return fmt.Errorf("mount move %s into / %s", rootfs, err)
+	}
+	if err := system.Chroot("."); err != nil {
+		return fmt.Errorf("chroot . %s", err)
+	}
+	return system.Chdir("/")
+}