fleetctl should sanity-check units

It's too easy to think you've set some parameters properly, but to accidentally misspell something and spend hours trying to hunt it down. fleetctl could read through the options set in [X-Fleet] and verify that they are all recognized before pushing units into the cluster.
Maybe we should just warn rather than fail here? Are there cases where a user might legitimately want to set something that their client does not recognise?