Known bugs in U-Net/NT4.0 :

* 'Unet' device may be unloaded before 'Tulip' device; causes system
  hang if Tulip is subsequently unloaded. Place lock on device unload
  if 'tulip' or other device-specific drivers are still present.

* Need to add rxBufferAllocSize/rxBufferUsedSize to devTulip.

* Unimplemented features:
  -- Pushing TLB-ousted Rx buffers onto user Rx FIFO unused (UTLB)
  -- Pushing Pfstart-rejected free buffers onto user Rx FIFO 
     (Zeitnet and Tulip)
  -- Rx callbacks (Zeitnet and Tulip)

* Zeitnet restricts Rx buffer sizes to single pages. Larger PDUs can be
  received, but multiple pages used.

* Zeitnet: Easy to get Rx FIFO overrun error, loopback with large packets
  (say, 4000 bytes).
