API, pt III: List Units

> 1. UnitState vs SystemdState?

Heh. Yup, now that we're pushing the Unit to a higher level abstraction, SystemdState seems a little clearer. Otherwise, what are we going to call the "fleet state"?

> 2. We have two relevant 'machine' values for a given unit, the target and the origin of the reported state. Should we report both of these through the API?

I think that could be useful.