fix github issue #594 - special attacks

by the Riders that only damage the hero, not other monsters.

Noticed and diagnosed by vultur-cadens.  Attacks dishing out the
damage type AD_DETH (Death), AD_FAMN (Famine), AD_PEST (Pestilence),
and also AD_DISE (Demogorgon, Juiblex, Scorpius) did 0 damage if
inflicted against monsters.  That made Death harmless to other
monsters.  The others have additional attacks (in the two Rider's
cases, only if both instances of their special attack hit on the
same move so that the second one is converted into a stun attack)
and could manage to kill other monsters.

The uhitm case can't happen.  I suspect that the mhitm case was
originally intentionally omitted as something which won't happen,
but that's just a guess.

Fixes #594

doc/fixes37.0

@@ -622,6 +622,8 @@ Izchak occasionally stocks wands/scrolls/spellbooks of light
 data tracking for #overview was mis-using u.urooms[] and after being in a
 	situation where hero was in multiple rooms at once, visiting other
 	levels might flag unvisisted rooms as having been visited
+special damage attacks by the Riders and by fatal-illness inflictors such as
+	Demogorgon did no damage against other monsters, only against the hero
 
 
 Fixes to 3.7.0-x Problems that Were Exposed Via git Repository