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)
That would mean refactoring the existing GetStats functionality to be aware that it is running inside systemd or not as this makes a difference to how the path to the subsystem stats files is derived. Existing way to know whether running under systemd or not is to call systemd.UseSystemd. Not sure how to do that without circular dependencies. Could create a new cgroups/stats package which could cater for both systemd & non-systemd without worrying about circular dependencies.

Alternatively, I could potentially turn the "data" struct in cgroups/fs/apply_raw.go into an interface with just a Path function to return the path to the subsystem stats files, but that would require changing the method signature for fs.GetStats.

I think I'm leaning toward the cgroups/stats pkg approach.

@vishh What do you think?