fleetctl: add list-unit-files command

This is functionally equivalent to `fleetctl list-units --fields
unit,hash,dstate' for now; it will start to make more sense as the registry
API and `fleetctl list-units` receive their own changes.

Not really sure on good descriptions so open to suggestions.
Yes, either we would need to have a consistent(-until-the-end-of-time) method for generating that representation on both side, or pass the actual unit file back.