detect flat cgroup mount, mount cgroup accordingly

If an existing cgroup mount is mounted flat, later cgroup mounts have to be
flat as well. Similarly, if there are existing mounts for individual
subsystems, later mounts have to be done the same. So, detect if there's an
existing flat mount, and mount ours flat as well, and just bind-mount each
subsystem.

This makes Garden compatible with more recent versions of LXC.