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.
Please be consistent with `<key>` -- I think the <key>.<key> is kinda redundant, and can be explained adequately in the docs (a key is just a dotted list of strings of a certain form).