net/portmapper: fall back to permanent UPnP leases if necessary

Some routers don't support lease times for UPnP portmapping; let's fall
back to adding a permanent lease in these cases. Additionally, add a
proper end-to-end test case for the UPnP portmapping behaviour.

Updates #9343

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I17dec600b0595a5bfc9b4d530aff6ee3109a8b12

net/portmapper/igd_test.go

@@ -16,6 +16,7 @@ import (
 
 	"tailscale.com/control/controlknobs"
 	"tailscale.com/net/netaddr"
+	"tailscale.com/syncs"
 	"tailscale.com/types/logger"
 )
 
@@ -25,6 +26,7 @@ type TestIGD struct {
 	upnpConn net.PacketConn // for UPnP discovery
 	pxpConn  net.PacketConn // for NAT-PMP and/or PCP
 	ts       *httptest.Server
+	upnpHTTP syncs.AtomicValue[http.Handler]
 	logf     logger.Logf
 	closed   atomic.Bool
 
@@ -126,8 +128,17 @@ func (d *TestIGD) stats() igdCounters {
 	return d.counters
 }
 
+func (d *TestIGD) SetUPnPHandler(h http.Handler) {
+	d.upnpHTTP.Store(h)
+}
+
 func (d *TestIGD) serveUPnPHTTP(w http.ResponseWriter, r *http.Request) {
-	http.NotFound(w, r) // TODO
+	if handler := d.upnpHTTP.Load(); handler != nil {
+		handler.ServeHTTP(w, r)
+		return
+	}
+
+	http.NotFound(w, r)
 }
 
 func (d *TestIGD) serveUPnPDiscovery() {