nspawn: rework /etc/machine-id handling

With this change we'll no longer write to /etc/machine-id from nspawn, as that
breaks the --volatile= operation, as it ensures the image is never considered
in "first boot", since that's bound to the pre-existance of /etc/machine-id.

The new logic works like this:

- If /etc/machine-id already exists in the container, it is read by nspawn and
  exposed in "machinectl status" and friends.

- If the file doesn't exist yet, but --uuid= is passed on the nspawn cmdline,
  this UUID is passed in $container_uuid to PID 1, and PID 1 is then expected
  to persist this to /etc/machine-id for future boots (which systemd already
  does).

- If the file doesn#t exist yet, and no --uuid= is passed a random UUID is
  generated and passed via $container_uuid.

The result is that /etc/machine-id is never initialized by nspawn itself, thus
unbreaking the volatile mode. However still the machine ID configured in the
machine always matches nspawn's and thus machined's idea of it.

Fixes: #3611

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