lxc: Work around lxc-start need for private mounts

lxc-start requires / to be mounted private, otherwise the changes
it does inside the container (both mounts and unmounts) will propagate
out to the host.

We work around this by starting up lxc-start in its own namespace where
we set / to rshared.

Unfortunately go can't really execute any code between clone and exec,
so we can't do this in a nice way. Instead we have a horrible hack that
use the unshare command, the shell and the mount command...