Adjust how some transport programs detect live input buffers to treat them as read only

Some of the transmit programs (at least cps_xmit and dix_xmit) determine if a input buffer is live by writing an invalid cycle to the buffer and waiting for a valid cycle number. This is causes issues with some things (tracking buffers through restarts via mbuf_probe) and is a bad philosophy of doing this, the input can and should be read only. These should be changed to look for a cycle change to check for live buffers.