Heretic: Fix broken door problem. (See E1M8 small/seed 1234, for example) 
         (FIXED; don't hardcode Doom-specific stuff like "blaze doors"!)

General: Make placing staircase a "soft" assertion; should it fail,
         we simply remake the one level instead of crashing Oblige. (DONE;
         in this particular case we remake the level)

