landmines and boulders

Make pushing a boulder onto a landmine share code with the trap case,
resulting in pits, waking sleepers, et al.
Don't leave a boulder suspended over the new pit, fill it.
Make sure any remaining boulder is placed on top of the pile.
If player sets off landmine, monsters killed are credited to/blamed on player.

src/explode.c

@@ -440,6 +440,11 @@ struct obj *obj;			/* only scatter this obj        */
 		    pline("%s breaks apart.",The(xname(otmp)));
 		    fracture_rock(otmp);
 		    place_object(otmp, sx, sy);	/* put fragments on floor */
+		    if ((otmp = sobj_at(BOULDER, sx, sy)) != 0) {
+			/* another boulder here, restack it to the top */
+			obj_extract_self(otmp);
+			place_object(otmp, sx, sy);
+		    }
 		} else {
 		    struct trap *trap;