Rename *_port to *_listen

We decided at a meeting today that socks_port and dns_port should be socks_listen and dns_listen respectively. @Diziet is taking this on.

(listen_socks and listen_dns would also be okay with me)

We also decided that it would be a good thing to build a backward-compatibility mechanism for this rename, so that we can be sure that backward-compatible renames are reasonable to implement in our system.