nspawn: always setup machine id

nspawn: always setup machine id

We check /etc/machine-id of the container and if it is already populated
we use value from there, possibly ignoring value of --uuid option from
the command line. When dealing with R/O image we setup transient machine
id.

Once we determined machine id of the container, we use this value for
registration with systemd-machined and we also export it via
container_uuid environment variable.

As registration with systemd-machined is done by the main nspawn process
we communicate container machine id established by setup_machine_id from
outer child to the main process by unix domain socket. Similarly to PID
of inner child.