Rather than each game backend file exporting a whole load of

Rather than each game backend file exporting a whole load of
functions and a couple of variables, now each one exports a single
structure containing a load of function pointers and said variables.
This should make it easy to support platforms on which it's sensible
to compile all the puzzles into a single monolithic application. The
two existing platforms are still one-binary-per-game.

git-svn-id: svn://svn.tartarus.org/sgt/puzzles@5126 cda61777-01e9-0310-a592-d414129be87e