Uniter Action(id string) API client/server

This commit adds the ability for Uniter clients to query Actions from
state.State by id.  The response includes a server error message if the
Action is not present on the state server.
OK, now I see more. Maybe. The unit tag we need to match is the *connected* unit, that you can find out from the auth thing. Putting that tag in the API call is meaningless. So, yes, extract the unit-that's-allowed-to-see from the action tag, that's correct; but you then need to check it against something different. And I'm now completely convinced there's no need for UnitTag in the api params.