PDP10,PDP11,VAX: Fix for DMC/DMR error which lost track of pending transmit buffers when timeouts happened during DDCMP protocol startup.  Fixes #109
Compiler suggested cleanups.
MAKEFILE: Added support to detect libSDL on host platforms which use /usr/pkg for OS supplied package installation location.  Fix #110
SCP: Fleshing out more of the hierarchical help content.