fix #H6489 - explosion double damage to ustuck

Report asked why u.ustuck takes double explosion damage, and concocting
a reason uncovered several inconsistencies.  Grabber takes double damage
for reaching into hero's spot, but only when that spot is within the
explosion's radius and only if hero isn't engulfed.  Poly'd hero takes
double damage if holding a monster which is hit by the explosion.

There are still multiple bugs here:  if the hero is grabbing a monster
which gets killed by the explosion, the fact that one was held is
forgotten by the time damage is inflicted upon the hero.  Just a messy
detail that I opted not to get bogged down in.  But much messier is
that grabber might be outside the explosion radius reaching into that
to hold grabbee, in which case no damage is inflicted.  Handling that
for out-of-range monster holding exploded hero shouldn't be very tough,
but handling it for out-of-range hero holding exploded monster could be
hard.  Anyway, it's more headache than I intend to tackle.

doc/fixes36.1

@@ -473,6 +473,11 @@ make Vlad slightly tougher
 reduce the amount of gold laying on the floor
 locked chests and large boxes contain more stuff
 make shopkeepers bill hero for burying merchandise
+monster holding hero takes double damage from an explosion for reaching into
+	adjacent spot, but it was being inflicted even when hero was swallowed
+	or beyond the explosion radius
+poly'd hero holding monster now takes double damage (if monster is within
+	explosion radious) instead of inflicting that upon the monster
 
 
 Fixes to Post-3.6.0 Problems that Were Exposed Via git Repository