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.
Hey, didn't we decide that Set and Update was the right model? It means we don't need to worry about the yucky non-copying (and the fact that ActionSetResults actually does *nothing* in this implementation).