PDP11: Fix to allow allow Q18 devices to be enabled on Qbus systems with memory <= 256KB.  Reported by Christian Gauger-Cosgrove.

The devices which are affected by this are: RK, HK, RM and VT.
Merge branch 'master' of https://github.com/simh/simh
SWTP: Added missing SWTP documentation files
Merge branch 'master' of http://github.com/simh/simh
PDP10: Change default bus address and vector for DMR device to be what the TOPS-10 expects (address 764000 and vector 610)
SCP: Initial merge of old style single line help with hierarchical help
SWTP: Fixes to segfault issue described in #103 (from Gene Irwin)

The simulated memory address block from 0x8000 thru 0x8FFF is defined to have address 0x8000 - 0x801F mapped to I/O ports and 0x8020 - 0x8FFF as NO RAM ALLOWED.  Code has been added to behave reasonably when references are made to 0x8020 - 0x8FFF.
Merge branch 'master' of https://github.com/jguillaumes/simh
Merge branch 'master' of https://github.com/simh/simh
Merge branch 'master' into BlinkenLights