Merge branch 'cgroups-systemd' into forking-run-systemd
cgroup: Add ReuseCurrent option to --unit-conf

This causes docker to not create a new transient unit for cgroup
containment and resource limits. Instead it will reuse the current
cgroup and apply resource limitations to it using SetUnitProperties.

This is interesting mostly in combination with --foreground, because
it allows you to have a systemd unit file like:

[Service]
ExecStart=docker run --foreground --unit-conf ReuseCurrent=true run image
MemoryLimit=100M

With the following properties:

* Any limits and other properties set up in the service file applies
  to both the docker client and the container "as one unit".
* There is only one unit representing the service
* You can kill the service via systemd which will kill both client and
  container.
* If the client dies systemd will (first nicely then violently) kill the
  other processes in the cgroup (i.e. the container).

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)