degesch: send PINGs with a timestamp

Instead of a username which is of no use to us.

Maybe some servers require the argument to be a target but the purpose of this
message is mainly to detect a failed connection at all, and that happens way
faster when you send some data.

degesch.c

@@ -3108,7 +3108,7 @@ on_irc_timeout (void *user_data)
 {
 	// Provoke a response from the server
 	struct server *s = user_data;
-	irc_send (s, "PING :%s", s->irc_user->nickname);
+	irc_send (s, "PING :%" PRIi64, (int64_t) time (NULL));
 }
 
 // --- Processing server output ------------------------------------------------