PDP11/DUP: Fixed broken modem behaviors. Added W3, W5 and W6 Jumper option features. Added/fixed missing API's for shared use (by KDP). Added loopback support.  Added Half-Duplex support.
PDP11/KMC: Removed NPR dup register access and replaced with fixed direct DUP APIs.
PDP11/DMC: Added crc validation check for incoming DDCMP packets
PDP11/KMC/DUP: Added receive packet available notification callback to minimize necessary polling for input traffic