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>.

include/hack.h

@@ -527,6 +527,13 @@ enum getobj_callback_returns {
     GETOBJ_SUGGEST  = 2,
 };
 
+/* constant passed to explode() for gas spores because gas spores are weird
+ * Specifically, this is an exception to the whole "explode() uses dobuzz types"
+ * system (the range -1 to -9 isn't used by it, for some reason), where this is
+ * effectively an extra dobuzz type, and some zap.c code needs to be aware of
+ * it.  */
+#define PHYS_EXPL_TYPE -1
+
 /*
  * option setting restrictions
  */