How to write portable IPv6 applications after rfc2553bis

for clients, just use examples in the slides.

for servers, discussions are still going on, but at this moment:
run two daemons
1st one opens an IPv6 socket, with the IPV6_ONLY option.
and THEN 2nd one opens an IPv4 socket.
unfortunately, getaddrinfo() may break portability...(sigh)