These PR does a few things. It ensures that the freezer cgroup is joined in the systemd driver. It also provides a public api for setting the freezer state via the cgroups package. Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael) Upstream-commit: 613f74c1fbbdc5e476d28974d1dbe3727033d083 Component: engine