net/netcheck: send dummy packet out to help airport extreme in hairpin check

At least the Apple Airport Extreme doesn't allow hairpin
sends from a private socket until it's seen traffic from
that src IP:port to something else out on the internet.

See https://github.com/tailscale/tailscale/issues/188#issuecomment-600728643

And it seems that even sending to a likely-filtered RFC 5737
documentation-only IPv4 range is enough to set up the mapping.
So do that for now. In the future we might want to classify networks
that do and don't require this separately. But for now help it.

I've confirmed that this is enough to fix the hairpin check on Avery's
home network, even using the RFC 5737 IP.

Fixes #188

net/netcheck/netcheck.go

@@ -671,6 +671,19 @@ func (c *Client) GetReport(ctx context.Context, dm *tailcfg.DERPMap) (*Report, e
 	}
 	defer rs.pc4Hair.Close()
 
+	// At least the Apple Airport Extreme doesn't allow hairpin
+	// sends from a private socket until it's seen traffic from
+	// that src IP:port to something else out on the internet.
+	//
+	// See https://github.com/tailscale/tailscale/issues/188#issuecomment-600728643
+	//
+	// And it seems that even sending to a likely-filtered RFC 5737
+	// documentation-only IPv4 range is enough to set up the mapping.
+	// So do that for now. In the future we might want to classify networks
+	// that do and don't require this separately. But for now help it.
+	const documentationIP = "203.0.113.1"
+	rs.pc4Hair.WriteTo([]byte("sets up mapping"), &net.UDPAddr{IP: net.ParseIP(documentationIP), Port: 12345})
+
 	if f := c.GetSTUNConn4; f != nil {
 		rs.pc4 = f()
 	} else {