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.
PDP10, PDP11, VAX780, VAX750, VAX730: Fixed DMC/DMR DDCMP implementation to be tolerant of packet corruption.
PDP10, PDP11, VAX: Fix DMC/DMR DDCMP implementation to not crash due to buffer exhaustion while generating NAK packets for missing packets.
PDP10, PDP11, VAX: Fixed Sync Data corruption Troll to actually discard transmit packets which it thinks it wants to.
PDP10, PDP11, VAX: Fixed DMC/DMR DDCMP implementation to only send a single NAK when a data packet arrives out of order.
PDP10, PDP11, VAX: Fix out of order packet numbers after processing some NAK data.  Also restart outgoing packet flow consistently if an output stall occurred.
PDP10, PDP11, VAX: Fix buffer leak while generating NAK messages
PDP11, VAX: Added Ethernet packet transmission throttling support to XQ and XU devices.

Migrated the XQ help to the hierarchical help model.  This is a work in progress which will eventually merge much from 0readme_ethernet.txt into the device help.
ETHER: Made LAN connections more robust under failing network conditions.

Avoid permanent network network hangs when a network transport starts to return errors for various reasons.
These hangs also result in at least one thread in a CPU bound loop attempting to read on a pcap connection which will never be useful again.

When transmit or receive errors occur, the invoking thread sleeps for 1 second and subsequent operations proceed.  When the total of read + write errors reaches a multiple of ETH_ERROR_REOPEN_THRESHOLD the current transport connection (usually pcap) is closed and re-opened after a ETH_ERROR_REOPEN_PAUSE second delay.  If the open succeeds, we're happy.  If it fails, then the link behaves as if it were never attached until some subsequent operator intervention is performed.

Activities which are known to induce this problems include (but are not limited to) when a simulator is running in a Windows Virtual Machine on a Hyper-V host system and the Hyper-V host system performs a SAVE and a subsequent Restart of the Guest Windows Virtual Machine.  This operation can occur due to specific operator requests or merely when the Hyper-V host system reboots.
Compiler indicated cleanup - Reported by Peter Schorn
Compiler suggested cleanups
Documentation update from Bob Supnik
AltairZ80: Improved Altair Minidisk support from Mike Douglas