Added ipv6_listening_ip option to override it from in6addr_any.

This way IPv6 services can be selectively enabled on one IP too.

This is useful if you don't want to firewall service, but just e.g. bind to bridge IPv6 address, or ::1 if you want to handle externally protocols elsewhere.

(I think multiple statically configured IPv6 addresses is not probably sensible, but this is a low-hanging fruit that I needed for a project.)