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.
How would we communicate the unit file over the API for `fleetctl cat`? How about we just say we have a method of creating a canonical representation of a unit file from a list of UnitOptions, and we do that to generate the hash both server- and client-side?