More serialisation changes: the game_aux_info structure has now been

More serialisation changes: the game_aux_info structure has now been
retired, and replaced with a simple string. Most of the games which
use it simply encode the string in the same way that the Solve move
will also be encoded, i.e. solve_game() simply returns
dupstr(aux_info). Again, this is a better approach than writing
separate game_aux_info serialise/deserialise functions because doing
it this way is self-testing (the strings are created and parsed
during the course of any Solve operation at all).

[originally from svn r6029]