Support ra-stateless IPv6 configuration

Support ra-stateless IPv6 configuration

This means that IPv6 will be auto-configured but DNS info
will be obtained via DHCPv6 requests.

With other words our router advertisements have the "O" flag set
and we add another handler that serves DHCPv6 requests by
passing DNS info.

Use a separate NFQUEUE to mangle the DHCPv6 traffic. Add another
ferm rule to achieve that.

This is done because Windows do not support RFC 6101:

http://tools.ietf.org/search/rfc6106
http://social.technet.microsoft.com/Forums/windowsserver/en-US/5757980a-5983-4efc-a5f3-27687b90fe41/does-win7-or-w2k8-server-support-rfc-6106?forum=ipv6

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>