Support multiple listen addresses for bind_address

[sfan5]

Right now you can either:

• listen on any address (incl. ipv6 if you use ipv6_server = 1)
• listen on a specific ipv4 or ipv6

We should allow for more flexibility by allowing binding to multiple addresses.

There are two approaches we could take.

1. permit at most one ipv4 and one ipv6 socket: this is easy to implement in the existing code since you can uniquely decide which packet should be received/sent from which socket. this will suffice in 99.9% use cases.
2. total freedom: this requires more changes to the connection code to track which peer is using which socket.

previous work: #2604