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.
what do you mean? I thought we established that this command was only for the desired state (essentially it's the manifest) - users need to use `list-units` for current states.