pkg: Set improvements
- add Equals
- be explicit that Set.Values() is not ordered
- improve test and documentation coverage
Merge pull request #693 from jonboulle/sets

pkg: sets are not ordered
pkg/set: rename equals to equal to be more logical
Merge pull request #696 from jonboulle/sets

pkg/set: follow my own thinking
test: fix typo
Merge pull request #698 from jonboulle/typo

test: fix typo
machine: refuse to accept empty machine ID
Merge pull request #697 from jonboulle/machineid

machine: refuse to accept empty machine ID
machine: use UnitManager interface instead of SystemdUnitManager
Merge pull request #701 from jonboulle/unitmgr

machine: use UnitManager interface instead of SystemdUnitManager
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.
Merge pull request #688 from jonboulle/list-unit-files

fleetctl: add list-unit-files command
agent: rename AgentState to AgentCache
agent: use AgentReconciler model to drive Agent