DEUNA: buffer chaining not implemened in backend

[wfjm]

The DEUNA supports buffer chaining, an Ethernet frame can be stored in a set of chained buffers.
2.11BSD doesn't use this feature, it always uses buffers large enough for a maximal size Ethernet frame.
However, DEC OS use buffer chaining.

The current backend doesn't support buffer chaining. No problem with 2.11BSD.
But this limitation prevents usage of Ethernet connectivity in DEC OS.

[wfjm]

Several mode settings are also not implemented in w11

  Mode bit    SimH  w11a   comment
  MODE_PROM    yes   yes   PROM: promiscous mode
  MODE_ENAL    yes    no   ENAL: enable all mcasts
  MODE_DRDC    yes    no   DRDC: disable data chaining
  MODE_TPAD    yes  part   TPAD: tx message padding enable
  MODE_ECT      no    no   ECT:  enable collision test
  MODE_DMNT    yes    no   DMNT: disable maintenance message
  MODE_INTL    yes    no   INTL: int loopback(DELUA)
  MODE_DTCR     no    no   DTCR:  disable tx CRC
  MODE_LOOP    yes    no   LOOP: int loopback
  MODE_HDPX     no    no   HDPX: half-duplex (DEUNA)

Therefore

• ENAL,DRDC,INTL,LOOP should be implemented
• TPAD should be fully implemented