CLI: unify, streamline, and refactor listener code

Right now we have two kinds of listeners: DNS and SOCKS. We should consider simplifying the logic that creates them a lot.

Some goals are:

• Eliminate duplicate code.
• Allow multiple listener ports of the same type.
• Allow listening on non-localhost addresses
• Fail with an error if the port binding fails for some reason other than "we don't support that address family."