go.net/ipv6: add ancillary data socket option binding tables

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/164540044
diff --git a/ipv6/sys_bsd.go b/ipv6/sys_bsd.go
index 226f574..fb6365b 100644
--- a/ipv6/sys_bsd.go
+++ b/ipv6/sys_bsd.go
@@ -14,6 +14,13 @@
 type sysSockoptLen int32
 
 var (
+	ctlOpts = [ctlMax]ctlOpt{
+		ctlTrafficClass: {sysIPV6_TCLASS, 4, marshalTrafficClass, parseTrafficClass},
+		ctlHopLimit:     {sysIPV6_HOPLIMIT, 4, marshalHopLimit, parseHopLimit},
+		ctlPacketInfo:   {sysIPV6_PKTINFO, sysSizeofInet6Pktinfo, marshalPacketInfo, parsePacketInfo},
+		ctlNextHop:      {sysIPV6_NEXTHOP, sysSizeofSockaddrInet6, marshalNextHop, parseNextHop},
+	}
+
 	sockOpts = [ssoMax]sockOpt{
 		ssoTrafficClass:        {ianaProtocolIPv6, sysIPV6_TCLASS, ssoTypeInt},
 		ssoHopLimit:            {ianaProtocolIPv6, sysIPV6_UNICAST_HOPS, ssoTypeInt},