action-set: a jujuc command for Actions to send values back to state

action-set permits action hooks to define an arbitrary map of params
which will be returned to the state server at the end of the Action,
or to specify --failed with an optional failure message.
...but I am kinda coming to hate the way we just return a reference to this internal map, and clients get to just write to it. It renders the paired SetActionResults call something of a joke...