The quick solution is to to unset the sysctl net.inet6.ip6.v6only using
sysctl -w net.inet6.ip6.v6only=0

Note that this behaviour is not enabled because it has security impact
(see for example
ftp://ftp.netbsd.org/pub/NetBSD/security/advisories/NetBSD-SA2006-016.txt.asc).

I think the best thing to do is probably to use a AF_INET6 socket to
connect to your AF_INET6 server. 

Another note on your code, you may use IN6ADDR_ANY_INIT or in6addr_any
(depending the context) to let your server binding on any available
iface.

I hope it will help.

Regards,
-- 
Arnaud Degroote
degroote@netbsd.org