SCP: Added hierarchical help capability (from Timothe Litt)
PDP11/VAX: Detect vector conflicts, SHOW IOSPACE and SHOW DEVICE improvements

From Timothe Litt:
  - Detect vector conflicts, SHOW IOSPACE
  - Detect conflicting vector assignments.
  - Correct show iospace display of high vector for multi-unit devices
  - Display vectors in DEV_RDX in show iospace
  - Ignore disabled devices when searching for conflict.
  - Improve device conflict reporting, Report both devices when address conflict is detected.

From Mark Pizzolato:
  - Added optional alternate radix output display for device address and vector values
    displayed by SHOW DEVICE.
    -H and -O switches select hex or octal output in addition to the default radix displayed
    by the normal simulator.
H316: Resurrecting the ARPAnet IMP (from Bob Armstrong)

This summer a group of us worked together to resurrect the original ARPAnet IMP software, and I’m now happy to say that the IMP lives again in simulation.    It’s possible to run the original IMP software on a modified version of the H316 simh and to set up a virtual network of simulated IMPs talking to each other.   IMP to IMP connections, which would have originally been carried over leased telephone lines, are tunneled over IP.  As far as we can tell, everything works pretty much as it did in the early 1970s.  IMPs are able to exchange routing information, console to console communications, network statistics, and they would carry host traffic if there were hosts on the network.  The hooks are in there to allow simh to support the IMP side of the 1822 host interface, and the next step would be to recover the OS for an ARPAnet era host and then extend the corresponding simulator to talk to the IMP simulation.
H316: Added IMP Listing file (from Bob Armstrong)
TMXR: Added option to use UDP transport for packet transport
Merge branch 'master' of github.com:simh/simh
Merge remote-tracking branch 'simh/master'