listener: ipv6only socket bind support According to RFC3493 and most Linux distributions, default value is to work in IPv4-mapped mode. If there is a requirement to bind same port on same ip addresses but different handlers for both IPv4 and IPv6, it is required to set IPV6_V6ONLY socket option to be sure that the code works as expected without affected by bindv6only sysctl setting in system. See an example working with this patch: https://gist.github.com/demirten/023008a63cd966e48b0ebcf9af7fc113 Closes: #640 (cherry-pick)