Add vendored version of github.com/guelfey/go.dbus

We need this to do systemd API calls.

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)
systemd: Add bindings for some systemd dbus calls

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

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

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Add -reuse-unit flag to allow reusing an existing systemd cgroup/unit

This flag, which works only if specified in combination with the
-foreground switch causes docker to not create a new transient unit
for the container, but rather reuse the current systemd unit for
resource limits.

This allows e.g. a systemd unit file to contain:
ExecStart=docker run -f -reuse-unit the-image

And docker will use the cgroup of the foo.service unit, allowing
nice integration with systemd.

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