Vendor github.com/godbus/dbus and github.com/coreos/go-systemd

We need this to do systemd API calls.

We also add the static_build tag to make godbus not use
os/user which is problematic for static builds.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
pkg/systemd: Drop our copy-pasted version of go-systemd/activation

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

This is a conversion of sd_booted() from libsystemd to go and checks
if the system was booted with systemd.

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

This implements cgroup.Apply() using the systemd apis.
We create a transient unit called "docker-$id.scope" that contains
the container processes. We also have a way to set unit specific
properties, currently only defining the Slice to put the
scope in.

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