vendor: add github.com/anacrolix/dms

vendor/golang.org/x/net/internal/socket/empty.s

@@ -0,0 +1,7 @@
+// Copyright 2018 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 darwin,go1.12
+
+// This exists solely so we can linkname in symbols from syscall.