Rename *_port to *_listen and provide ListenConfig type for API
As per #502 (closed).
The good news is that, as planned, I haven't painted us into a corner. It is entirely possible to provide compatibility like this.
The bad news is that it is a slight palaver; particularly, one needs to handle (and test!) all the permutations of things being specified the old way, or the new way, or both, via serde and via the API.
Edited by Ian Jackson