port-serial: allow deciding whether the command is queued last or run next

By default all the commands we were sending through the serial port
were added at the tail of the pending queue, but we may want to queue
them at the head in very specific cases (e.g. while sending an SMS).

plugins/mbm/mm-broadband-modem-mbm.c

@@ -1262,7 +1262,8 @@ gps_enabled_ready (MMBaseModem *self,
             mm_port_serial_command (MM_PORT_SERIAL (gps_port),
                                     buf,
                                     3,
-                                    FALSE,
+                                    FALSE, /* never cached */
+                                    FALSE, /* always queued last */
                                     NULL,
                                     NULL,
                                     NULL);