add listen.send_recv_error config option (#670)

By default, Nebula replies to packets it has no tunnel for with a `recv_error` packet. This packet helps speed up re-connection
in the case that Nebula on either side did not shut down cleanly. This response can be abused as a way to discover if Nebula is running
on a host though. This option lets you configure if you want to send `recv_error` packets always, never, or only to private network remotes.
valid values: always, never, private

This setting is reloadable with SIGHUP.

main.go

@@ -306,6 +306,8 @@ func Main(c *config.C, configTest bool, buildVersion string, logger *logrus.Logg
 
 		ifce.RegisterConfigChangeCallbacks(c)
 
+		ifce.reloadSendRecvError(c)
+
 		go handshakeManager.Run(ctx, ifce)
 		go lightHouse.LhUpdateWorker(ctx, ifce)
 	}