nspawn: Communicate determined UID shift to parent

nspawn: Communicate determined UID shift to parent

There is logic to determine the UID shift from the file-system, rather
than having it be explicitly passed in.

However, this needs to happen in the child process that sets up the
mounts, as what's important is the UID of the mounted root, rather than
the mount-point.

Setting up the UID map needs to happen in the parent becuase the inner
child needs to have been started, and the outer child is no longer able
to access the uid_map file, since it lost access to it when setting up
the mounts for the inner child.

So we need to communicate the uid shift back out, along with the PID of
the inner child process.

Failing to communicate this means that the invalid UID shift, which is
the value used to specify "this needs to be determined from the file
system" is left invalid, so setting up the user namespace's UID shift
fails.