Add systemd stats

Discussed in https://github.com/dotcloud/docker/pull/5424.

This is basically a refactoring of the existing fs cgroups stats functionality, using same functionality but with different subsystem paths for systemd.

Docker-DCO-1.1-Signed-off-by: Jimmi Dyson <jimmidyson@gmail.com> (github: jimmidyson)
What about we change the per-controller GetStats() to take the path (since all they do is get the path from their hierarchy anyways). This would look a lot like what you have today, but moving up the d.path(<subsystem>) into the fs.GetStats(). Then we'd have systemd implement its own GetStats() that changes the path. I guess similar to what you have today, but also using the subsystems as is done in fs.GetStats().