Add `bindaddr6` to cleanly separate between ipv4 & ipv6 bind addresses

[Jupeyy]

For domains it can make sense to have them both resolve to ipv4 & ipv6. But for ips, it's rather weird to use a ipv4 to bind to a ipv6 socket.

To make this separation clear, the bindaddr should be specified for both individually

[timakro]

Potentially also ec_bindaddr6