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.
Merge pull request #688 from jonboulle/list-unit-files

fleetctl: add list-unit-files command