Alter the `Octagon' board preset so that instead of presenting you

with the obvious central hole it presents you with a randomly chosen
one of twelve other holes. The reason is, the central-hole starting
position is provably insoluble (proof given in comments), so instead
we pick from the ones that are actually possible.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@6083 cda61777-01e9-0310-a592-d414129be87e