BUG/MINOR: listener: use sockaddr_in6 for IPv6

BUG/MINOR: listener: use sockaddr_in6 for IPv6

A copy-paste bug between {tcp,udp}{4,6}_add_listener() resulted in
using a struct sockaddr_in to set the TCP/UDP port while it ought to
be a struct sockaddr_in6. Fortunately, the port has the same offset
(2) in both so it was harmless. A cleaner way to proceed would be
to have a set_port function exported by the address family layer.

This needs to be backported to 2.3.