Large polyforms unequip cloaks rather than destroying them

This commit is intended to achieve two things: a) making Vlad's
throne a little less dangerous, b) making polymorph traps less
dangerous for players who don't have magic resistance. (At present,
uncontrolled polymorph is too dangerous in the late game for most
players to consider risking it, so they take care to avoid things
that have even a small chance of it, cutting out a lot of potential
strategies. Toning down polymorphs should make them happen more
often by increasing the chance that players will be willing to go
without precautions.)

Body armor is still destroyed (with the existing exception of
dragon armor), in order to mostly preserve the balance effect of
polymorph traps in the early game.

doc/fixes3-7-0.txt

@@ -1565,6 +1565,7 @@ scroll of enchant armor formula has changed (in particular, magical armor now
         uncursed scrolls can sometimes enchant by more than one point)
 dragonhide can rot
 throwing ammo without a launcher produces a message
+polymorphing into a large polyform unequips rather than destroying cloaks
 
 
 Fixes to 3.7.0-x General Problems Exposed Via git Repository