Some monsters can eat through iron bars

Any monster with rusting or corrosion attack can eat through
the bars. This includes rust monsters, grey oozes, and black puddings.

Original patch by Malcolm Ryan

src/mondata.c

@@ -285,6 +285,7 @@ struct permonst *mptr;
 {
     return (boolean) (passes_walls(mptr) || amorphous(mptr) ||
 		      unsolid(mptr) || is_whirly(mptr) || verysmall(mptr) ||
+		      dmgtype(mptr, AD_CORR) || dmgtype(mptr, AD_RUST) ||
 		      (slithy(mptr) && !bigmonst(mptr)));
 }