Support for these was removed in 3721ff82ff as the feature is
inherently insecure. Checkpoints imply an extremely privileged
operation that is incompatible with the constraints of a
`podman run` command. However, we should still tell folks that
it no longer works with a clear error message. This is done
during Specgen processing, server side, which should be safer
than the previous client-side processing for these.
<MH: Fixed cherry-pick conflicts>
Signed-off-by: Matt Heon <matthew.heon@pm.me>
24706290d5