net/tsdial: add SystemDial as a wrapper on netns.Dial

The connections returned from SystemDial are automatically closed when
there is a major link change.

Also plumb through the dialer to the noise client so that connections
are auto-reset when moving from cellular to WiFi etc.

Updates #3363

Signed-off-by: Maisem Ali <maisem@tailscale.com>

tsnet/tsnet.go

@@ -105,6 +105,7 @@ func (s *Server) Close() error {
 	s.shutdownCancel()
 	s.lb.Shutdown()
 	s.linkMon.Close()
+	s.dialer.Close()
 	s.localAPIListener.Close()
 
 	s.mu.Lock()