API, pt III: List Units

https://github.com/bcwaldon/fleet/commit/ab45c0287a3a9d1544261b2f699e8abb6f10e2c5

A couple of things:
1. UnitState vs SystemdState?
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?