go.net/ipv6: add sticky socket option binding

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/165160043

ipv6/sockopt.go

+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ipv6
+
+// Sticky socket options
+const (
+	ssoTrafficClass        = iota // header field for unicast packet, RFC 3542
+	ssoHopLimit                   // header field for unicast packet, RFC 3493
+	ssoMulticastInterface         // outbound interface for multicast packet, RFC 3493
+	ssoMulticastHopLimit          // header field for multicast packet, RFC 3493
+	ssoMulticastLoopback          // loopback for multicast packet, RFC 3493
+	ssoReceiveTrafficClass        // header field on received packet, RFC 3542
+	ssoReceiveHopLimit            // header field on received packet, RFC 2292 or 3542
+	ssoReceivePacketInfo          // incbound or outbound packet path, RFC 2292 or 3542
+	ssoReceivePathMTU             // path mtu, RFC 3542
+	ssoPathMTU                    // path mtu, RFC 3542
+	ssoChecksum                   // packet checksum, RFC 2292 or 3542
+	ssoICMPFilter                 // icmp filter, RFC 2292 or 3542
+	ssoJoinGroup                  // any-source multicast, RFC 3493
+	ssoLeaveGroup                 // any-source multicast, RFC 3493
+	ssoMax
+)
+
+// Sticky socket option value types
+const (
+	ssoTypeInt = iota + 1
+	ssoTypeInterface
+	ssoTypeICMPFilter
+	ssoTypeMTUInfo
+	ssoTypeIPMreq
+)
+
+// A sockOpt represents a binding for sticky socket option.
+type sockOpt struct {
+	level int // option level
+	name  int // option name, must be equal or greater than 1
+	typ   int // option value type, must be equal or greater than 1
+}