libcontainer/cgroup: Use raw access to set up and join the devices cgroup

The systemd support for the devices cgroup lacks two required features:
 * Support for wildcards to allow mknod on any device
 * Support for wildcards to allow /dev/pts support

The second is available in more recent systemd as "char-pts", but not in e.g. v208 which is in wide use.

Additionally, the current approach of letting systemd set up the devices cgroup and then adding
some devices to it doesn't work, because some times systemd (at least v208) re-initializes
the devices cgroup, overwriting our custom devices. See https://github.com/dotcloud/docker/issues/6009
for the details.

When wildcarded mknod support is available in systemd we should implement a pure systemd version,
but we need to keep the old one around for backwards compat.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #6175 from alexlarsson/systemd-use-raw-devices

libcontainer/cgroup: Use raw access to set up and join the devices cgroup