job/registry: split out new Job/Unit structures

- add new ScheduledUnit struct to represent the state of a Job's
  scheduling according to fleet (corresponds directly to `fleetctl
  list-unit-schedule`)
- add new JobUnit to encapsulate a unit file (corresponds to `fleetctl
  list-unit-files`)
- add associated Registry methods: JobUnits() and ScheduledUnits()
fleetctl: update commands to use new registry calls
fleetctl/registry: rename functions
  - fleetctl list-unit-schedule ==> fleetctl show-schedule
  - registryScheduledUnits() ==> registry.Schedule()
docs: update fleetctl doc to reflect new commands
registry: add more test coverage, sort UnitStates