[HECnet] PyDECnet update

Gentlepeople, I committed rev 560, which may be of interest to a number of you. It's basically a change of internal machinery, no functional changes. The main ones are: 1. Restructured the point to point (DDCMP and Multinet) implementations. Error recovery is much cleaner now and should be faster. Retry for persistent problems has holdoff on it that starts out pretty fast and slows down to (typically) one retry per two minutes. 2. Ethernet in PCAP mode now uses the PCAP filter mechanism to request only the frames we want. This means the Python code no longer spends a bunch of CPU time tossing away non-DECnet frames. If your machine runs other traffic in significant quantities, as mine does, this can be quite useful. 3. Improve the performance of packet logging. This code has been running on PYTHON for several days now without problems (except for one bug that has been fixed). You're invited to give it a try; as always, problem reports or other comments are welcome. paul