Various fixes and cleanups suggested by Ben Hutchings:

Various fixes and cleanups suggested by Ben Hutchings:
 - clarified wording of messages in validate_params(), including in
   particular a correction from `< 255' to `<= 255'
 - fixed random_upto() in game generation which caused the maximum
   number of balls never to be used when there was uncertainty
 - fixed widespread miscalculation of rectangular-array indices
   (multiplication by h instead of w, which would have broken
   non-square grids rather profoundly)
 - corrected an ANSI namespace violation
 - removed real functionality from the inside of assert()
   statements, so that the game should still work when compiled
   -DNDEBUG
 - couple of unnecessary linear-time loops removed.

[originally from svn r6149]