Exploding spheres cause real explosions

Despite active explosion attacks being called explosions in-game,
they only affected a single target, and were handled differently
from actual explosions. Make them do an actual explosion instead.
This should make spheres more interesting and inspire different
tactics handling them.

Because spheres deal more damage on average and can destroy items
in their explosions, their difficulty has been increased slightly.

Polyselfed hero exploding won't cause elemental damage to their
own gear.

Originally from xNetHack by copperwater <aosdict@gmail.com>.

src/mon.c

@@ -2522,12 +2522,7 @@ corpse_chance(
                 return FALSE;
             }
 
-            Sprintf(g.killer.name, "%s explosion",
-                    s_suffix(pmname(mdat, Mgender(mon))));
-            g.killer.format = KILLED_BY_AN;
-            explode(mon->mx, mon->my, -1, tmp, MON_EXPLODE, EXPL_NOXIOUS);
-            g.killer.name[0] = '\0';
-            g.killer.format = 0;
+            mon_explodes(mon, &mdat->mattk[i]);
             return FALSE;
         }
     }