libcontainer: Initial version of cgroups support

This is a minimal version of raw cgroup support for libcontainer.
It has only enough for what docker needs, and it has no support
for systemd yet.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #10 from alexlarsson/cgroups

libcontainer: Initial version of cgroups support