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)
Would that mean that each individual subsystem type would have to be exported from fs? I might be missing something though - go n00b :)