Use a dedicated copy function to copy dsfs.

Previously we were duplicating the contents of a dsf using straight-up
memcpy. Now there's a dsf_copy function wrapping the same memcpy.

For the moment, this still has to take a size parameter, because the
size isn't stored inside the dsf itself. But once we make a proper
data type, it will be.

signpost.c

@@ -482,7 +482,7 @@ static void dup_game_to(game_state *to, const game_state *from)
     memcpy(to->next, from->next, to->n*sizeof(int));
     memcpy(to->prev, from->prev, to->n*sizeof(int));
 
-    memcpy(to->dsf, from->dsf, to->n*sizeof(int));
+    dsf_copy(to->dsf, from->dsf, to->n);
     memcpy(to->numsi, from->numsi, (to->n+1)*sizeof(int));
 }