libcontainer: Use MS_PRIVATE instead of MS_SLAVE

Now that we unmount all the mounts from the global namespace we can
use a private namespace rather than a slave one (as we have no need
for unmounts of inherited global mounts to propagate into the
container).

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #4465 from alexlarsson/private

libcontainer: Use MS_PRIVATE instead of MS_SLAVE