lib/connections: Support setting traffic class (fixes #3790)

vendor/golang.org/x/net/ipv4/payload.go

@@ -0,0 +1,18 @@
+// Copyright 2012 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 ipv4
+
+import "net"
+
+// BUG(mikio): On Windows, the ControlMessage for ReadFrom and WriteTo
+// methods of PacketConn is not implemented.
+
+// A payloadHandler represents the IPv4 datagram payload handler.
+type payloadHandler struct {
+	net.PacketConn
+	rawOpt
+}
+
+func (c *payloadHandler) ok() bool { return c != nil && c.PacketConn != nil }