Commit b6967fa8

 moved the functionality of managing cgroup devices
into a separate package, and decoupled libcontainer/cgroups from it.

Yet, some software (e.g. cadvisor) may need to use libcontainer package,
which imports libcontainer/cgroups/devices, thus making it impossible to
use libcontainer without bringing in cgroup/devices dependency.

In fact, we only need to manage devices in runc binary, so move the
import to main.go.

The need to import libct/cg/dev in order to manage devices is already
documented in libcontainer/cgroups, but let's
 - update that documentation;
 - add a similar note to libcontainer/cgroups/systemd;
 - add a note to libct README.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>