Having played Keen a bit following the clue-generation fix in r9165,

Having played Keen a bit following the clue-generation fix in r9165,
I've decided that the extremely low density of one-option
multiplication clues is not a universally good idea after all: it
seems to me to make puzzles _quantitatively_ harder, even if Keen's
difficulty-level system can't see any difference in the set of modes
of reasoning required at least once to solve the grid.

So I've readjusted the clue selection, so that multiplicative clues
with only one workable pair of factors are restored to 'good' status
at Normal difficulty level and below, and only considered less-than-
fully-desirable at Hard and above. I think that's a reasonable
compromise.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@9170 cda61777-01e9-0310-a592-d414129be87e