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>

ipn/ipnlocal/local.go

@@ -1036,6 +1036,7 @@ func (b *LocalBackend) Start(opts ipn.Options) error {
 		LinkMonitor:          b.e.GetLinkMonitor(),
 		Pinger:               b.e,
 		PopBrowserURL:        b.tellClientToBrowseToURL,
+		Dialer:               b.Dialer(),
 
 		// Don't warn about broken Linux IP forwarding when
 		// netstack is being used.