simplify achievement tracking for special objects

This turned out to be a lot more work than I anticipated, but it is
definitely simpler (other than having #wizmakemap take achievements
away if you replace the level that contains the 'prize', which wasn't
handled before).

I cheated and made Mine's End into a no-bones level because the new
flagging scheme for luckstone, bag, and amulet can't carry over from
one game to another.  It probably should have been no-bones all along.
Sokoban didn't have this issue because it's already no-bones.

Existing save files are invalidated.

src/decl.c

@@ -638,8 +638,6 @@ const struct instance_globals g_init = {
     UNDEFINED_VALUE, /* ysize */
     FALSE, /* splev_init_present */
     FALSE, /* icedpools */
-    0, /* mines_prize_count */
-    0, /* soki_prize_count */
     { UNDEFINED_PTR }, /* container_obj */
     0, /* container_idx */
     NULL, /* invent_carrying_monster */