Cleanup systemd cgroup code

The current paths for the different systemd cgroup subsystems that
systemd manages and that we have to manage are very inconsistent.  This
patch cleans up those differences and allows consistent paths to be
used.

Signed-off-by: Michael Crosby <michael@docker.com>
Merge pull request #4 from crosbymichael/cgroup-enter

Cleanup systemd cgroup code