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>

derp/derphttp/derphttp_test.go

@@ -15,9 +15,14 @@ import (
 	"time"
 
 	"tailscale.com/derp"
+	"tailscale.com/net/netns"
 	"tailscale.com/types/key"
 )
 
+func init() {
+	netns.TestOnlySkipPrivilegedOps()
+}
+
 func TestSendRecv(t *testing.T) {
 	const numClients = 3
 	var serverPrivateKey key.Private