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'm a bit worried about exposing a hash moving forward. If we move over to using a list of unit file objects to represent a unit file, it becomes less clear what a hash means. You won't be able to simply run `md5sum foo.service` and compare that to the data stored in the Registry.