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.
I don't really follow. How are you suggesting we distinguish between multiple unit states with the same name? How do users tell what the latest/definitive is?