bus: implicitly set no_reply flag on outgoing messages if the serial number is not kept

If nobody keeps the serial number of an outgoing message we know that
nobody expects an answer to it, so set the no_reply flag accordingly.

src/libsystemd-bus/sd-bus.c

@@ -1057,6 +1057,11 @@ int sd_bus_send(sd_bus *bus, sd_bus_message *m, uint64_t *serial) {
         if (!m)
                 return -EINVAL;
 
+        /* If the serial number isn't kept, then we know that no reply
+         * is expected */
+        if (!serial && !m->sealed)
+                m->header->flags |= SD_BUS_MESSAGE_NO_REPLY_EXPECTED;
+
         r = bus_seal_message(bus, m);
         if (r < 0)
                 return r;

src/libsystemd-bus/sd-bus.h

@@ -33,7 +33,6 @@
  * - add page donation logic
  * - api for appending/reading fixed arrays
  * - always verify container depth
- * - implicitly set no_reply when a message-call is sent an the serial number ignored
  * - handle NULL strings nicer when appending
  * - merge busctl into systemctl or so?
  * - add object handlers