net/tstun,wgengine/{.,netstack}: handle UDP magicDNS traffic in netstack

This change wires netstack with a hook for traffic coming from the host
into the tun, allowing interception and handling of traffic to quad-100.

With this hook wired, magicDNS queries over UDP are now handled within
netstack. The existing logic in wgengine to handle magicDNS remains for now,
but its hook operates after the netstack hook so the netstack implementation
takes precedence. This is done in case we need to support platforms with
netstack longer than expected.

Signed-off-by: Tom DNetto <tom@tailscale.com>

tsnet/tsnet.go

@@ -196,12 +196,12 @@ func (s *Server) start() error {
 		return err
 	}
 
-	tunDev, magicConn, ok := eng.(wgengine.InternalsGetter).GetInternals()
+	tunDev, magicConn, d, ok := eng.(wgengine.InternalsGetter).GetInternals()
 	if !ok {
 		return fmt.Errorf("%T is not a wgengine.InternalsGetter", eng)
 	}
 
-	ns, err := netstack.Create(logf, tunDev, eng, magicConn, s.dialer)
+	ns, err := netstack.Create(logf, tunDev, eng, magicConn, s.dialer, d)
 	if err != nil {
 		return fmt.Errorf("netstack.Create: %w", err)
 	}