* the InitConfigBuilder non-miette error goes away
* creating an InitConfig is no longer faillible for no reason
* the "builder" style is consistent between the two config structs
Also change the concept of a completion handler to instead sending a
synthetic "process completed" event down the same path as usual.
That makes handling completion the job of the action handler, but also
means it's immediately possible to launch a process or do an action in
response to the process completing. Win win!