Implement selection of game seeds, by reusing the config box

mechanism I've just invented (the midend handles the standard game
selection configuration). Each game is now required to validate its
own seed data before attempting to base a game on it and potentially
confusing itself.

[originally from svn r4186]

nullgame.c

@@ -81,6 +81,11 @@ char *new_game_seed(game_params *params)
     return dupstr("FIXME");
 }
 
+char *validate_seed(game_params *params, char *seed)
+{
+    return NULL;
+}
+
 game_state *new_game(game_params *params, char *seed)
 {
     game_state *state = snew(game_state);