Close multicast listeners on stop

On Android, if Yggdrasil is disabled, but multicast listeners are not
closed, it's stuck in some kind of live-lock:

  Connected inbound: 202:14b7:a:b:c:d:e:f@[a::b:c:d:e%wlan0]:37351, source [f::e:d:c:b%wlan0]:41127
  Disconnected inbound: 202:14b7:a:b:c:d:e:f@[a::b:c:d:e%wlan0]:37351, source [f::e:d:c:b%wlan0]:41127; error: ErrClosed
  ...

And the only way to resolve this issue is to restart the entire
process.

src/multicast/multicast.go

@@ -142,6 +142,12 @@ func (m *Multicast) _stop() error {
 	if m.sock != nil {
 		m.sock.Close()
 	}
+	for name, info := range m._listeners {
+		info.listener.Close()
+		m.log.Debugln("Stop multicasting on ", name)
+	}
+	clear(m._listeners)
+
 	return nil
 }