ETHER, PDP11, VAX: Added a built-in loopback agent which all ethernet devices can leverage to detect MAC address conflicts.

Some of this behavior was previously available in the pdp11_xq simulation for the DELQA device variants, but not the DEQNA.  This adds the behavior for all ethernet devices.

The helps to avoid MAC address conflicts since the Ethernet physical address is completely user settable, and starts with a static value.  Multiple simulators on the same LAN will have conflicting addresses if they don't specifically set unique values.
PDP10, PDP11, VAX780, VAX750, VAX730: Fix DMC/DMR simulation to generate NAK messages with the correct reason for CRC errors.