[refactor] use only sockaddr for storing listener port
listen to ipv6 addresses (conf directive `port` has been changed to `listen`)
print the reason in case listen(2) returns an error
set IPV6_V6ONLY (see http://funcptr.net/2012/08/07/ipv6----getaddrinfo%28%29-and-bind%28%29-ordering-with-v6only/)
log IPv6 addresses (preserve addr using sockaddr_storage)
add support for IPv6 (merge branch 'kazuho/expr/ipv6')