PDP10, PDP11, VAX: Added the DDCMP corruption troll to the DDCMP layer and removed it from the KDP device.

Now all DDCMP capable devices KDP, DUP and DMC/DMR have access to packet corruption generation to simulate real world imperfect data lines for protocol testing.
SCP: Cleaned up the hierarchical help APIs to follow the same indentation style used in the rest of scp.c.  Fixed the help prompt to avoid adding an extra level to the prompt when the current level has no children.
PDP10: Correct time interpolation when reading the timebase and the interval calculation when setting the clock tick interval.  Fixes issue #108 (from Jordi Guillaumes)