fix #H7536 - pet vampire behavior

Vampires tend to take vampire bat form and stay that way, unless/until
there's a closed door they want to pass in which case they change to
fog cloud form.  Those shifted forms are weak, so pet vampires tend
not to attack other monsters, and if they don't take damage, they
won't change to vampire form.  So, when comparing relative strength of
self and foe while deciding whether to attack another monster, treat
their own strength in weak form as if in vampire form, making them be
more aggressive.

Hostile vampires shouldn't need any comparable change.  They don't use
relative strengths when deciding whether to attack something.

doc/fixes36.2

@@ -197,6 +197,7 @@ avoid potential buffer overflow if object with very long name knocks other
 clairvoyance would show trap instead of a monster on/in that trap, which was
 	intentional, but when clairvoyance finished the monster wasn't shown
 nurse corpse/tin chance to convey poison resistance when eaten was not honored
+make tame vampires be more aggressive when shifted to bat/fog/wolf form
 
 
 Fixes to Post-3.6.1 Problems that Were Exposed Via git Repository