net/netns: set the bypass socket mark on linux.

This allows tailscaled's own traffic to bypass Tailscale-managed routes,
so that things like tailscale-provided default routes don't break
tailscaled itself.

Progress on #144.

Signed-off-by: David Anderson <danderson@tailscale.com>

net/netns/netns_default.go

@@ -0,0 +1,14 @@
+// Copyright (c) 2020 Tailscale Inc & 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 !linux
+
+package netns
+
+import "syscall"
+
+// control does nothing to c.
+func control(network, address string, c syscall.RawConn) error {
+	return nil
+}