Multicast interface detection and shutdown tweaks

May help with #1173.

src/core/link.go

@@ -422,7 +422,10 @@ func (l *links) listen(u *url.URL, sintf string) (*Listener, error) {
 	li := &Listener{
 		listener: listener,
 		ctx:      ctx,
-		Cancel:   cancel,
+		Cancel: func() {
+			cancel()
+			_ = listener.Close()
+		},
 	}
 
 	var options linkOptions

src/multicast/multicast.go

@@ -190,6 +190,8 @@ func (m *Multicast) _getAllowedInterfaces() map[string]*interfaceInfo {
 		switch {
 		case iface.Flags&net.FlagUp == 0:
 			continue // Ignore interfaces that are down
+		case iface.Flags&net.FlagRunning == 0:
+			continue // Ignore interfaces that are not running
 		case iface.Flags&net.FlagMulticast == 0:
 			continue // Ignore non-multicast interfaces
 		case iface.Flags&net.FlagPointToPoint != 0: