x/net/ipv6: enable IPV6_NEXTHOP, IPV6_PATHMTU ancillary data options for BSD variants

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/174310043
diff --git a/ipv6/sys_bsd.go b/ipv6/sys_bsd.go
index 5e10381..75a8863 100644
--- a/ipv6/sys_bsd.go
+++ b/ipv6/sys_bsd.go
@@ -21,6 +21,7 @@
 		ctlHopLimit:     {sysIPV6_HOPLIMIT, 4, marshalHopLimit, parseHopLimit},
 		ctlPacketInfo:   {sysIPV6_PKTINFO, sysSizeofInet6Pktinfo, marshalPacketInfo, parsePacketInfo},
 		ctlNextHop:      {sysIPV6_NEXTHOP, sysSizeofSockaddrInet6, marshalNextHop, parseNextHop},
+		ctlPathMTU:      {sysIPV6_PATHMTU, sysSizeofIPv6Mtuinfo, marshalPathMTU, parsePathMTU},
 	}
 
 	sockOpts = [ssoMax]sockOpt{