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

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

@@ -0,0 +1,13 @@
+// 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.
+
+// +build nacl plan9
+
+package ipv4
+
+var (
+	ctlOpts = [ctlMax]ctlOpt{}
+
+	sockOpts = [ssoMax]sockOpt{}
+)