agent/reconcile: remove State from desired state

The State field is not required or useful when calculating the desired
state for what an agent is running; rather, we simply use the
TargetState/DesiredState of a Unit.
Merge pull request #785 from jonboulle/state

agent/reconcile: remove State from desired state
job: add IsGlobal to job.Unit/fleetctl, and associated test coverage