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.
"List the units that exist in the cluster"
"List the current state of unit files in the cluster"

idk