Implement systemd support for freezer

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)
Merge pull request #6125 from crosbymichael/systemd-freezer

Implement systemd support for freezer