Lower Write service Procedure

muxlwsrv, the lower (linked) queue write service procedure is scheduled as a result of flow control subsiding downstream (it is back-enabled).

static int muxlwsrv(queue_t *q)
{
	int i;

	for (i = 0; i < mux_cnt; i++)
			if (mux_mux[i].qptr && mux_mux[i].qptr->q_first)
				qenable(mux_mux[i].qptr);
	return (0);
}

muxlwsrv steps through all possible upper queues. If a queue is active and there are messages on the queue, then its upper write service procedure is enabled through qenable.