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

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

@@ -0,0 +1,11 @@
+// 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.
+
+// +build nacl plan9
+
+package ipv4
+
+func setInt(s uintptr, opt *sockOpt, v int) error {
+	return errOpNoSupport
+}