research MOVE_MOUNT_BENEATH for LXCFS daemon restarts/crashes

[mihalicyn]

We might consider using MOVE_MOUNT_BENEATH feature to perform necessary mounts under existing /proc/loadavg, /proc/meminfo and other LXCFS bindmounts. It still will be a quite expensive operation, because probably we will need to iterate over all "containers", but it worth investigation, cause it can be a live saver in cases when LXCFS daemon suddenly crashes. There days folks have to reboot the entire machine to make things back to normal which is not good.