Very small hunks

[KMP at MIT-MC (Kent M. Pitman)]

Well, I've never used 1-length hunks so (NIL . <thing>) for hunk1's in
MAKHUNK[NIL] mode seems best to me (given that CXR[0] is what it is).
If no one else objects, the change seems fine. Having anything but 
#777777 in that slot is an improvement.