Add --opt arguments for drivers

In order to handle special configuration for different drivers we
make the Config field a map to string array. This lets
us use it for lxc, by using the "lxc" key for those, and we can
later extend it easily for other backend-specific options.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
cgroups: Splity out Apply/Cleanup to separate file/interface

This leaves only the generic cgroup helper functions in cgroups.go and
will allow easy implementations of other cgroup managers.

This also wires up the call to Cleanup the cgroup which was missing
before.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
cgroups: Join groups by writing to cgroups.procs, not tasks

cgroups.procs moves all the threads of the process, and "tasks" just
the one thread. I believe there is a risk that we move the main thread,
but then we accidentally fork off one of the other threads if the go
scheduler randomly switched to another thread. So, it seems safer (and
more correct) to use cgroups.procs.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
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 vendored version of github.com/coreos/go-systemd

We need this to do systemd API calls.

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)