nspawn: figure out cgroup mode *after* mounting image

If we operate on a disk image (i.e. --image=) then it's pointless to
look into the mount directory before it is actually mounted to see which
systemd version is running inside...

Unfortunately we only mount the disk image in the child process, but the
parent needs to know the cgroup mode, hence add some IPC for this
purpose and communicate the cgroup mode determined from the image back
to the parent.