Iron footwear and traps balance fixes

Any sort of iron footwear now protects from traps, and polymorph
traps change one sort of iron footwear into a different sort of
iron footwear (removing any incentive to try to farm them, because
you will see both options very quickly).

doc/fixes3-7-0.txt

@@ -2917,7 +2917,7 @@ add "make fetch-docs" to download pre-formatted documentation
 monsters will use throw-and-return weapons such as an aklys
 archeologists can identify scrolls by deciphering their labels
 monster destroy armor -spell first erodes armor
-iron shoes protect the wearer against certain floor-based traps
+iron footwear protects the wearer against certain floor-based traps
 ring of stealth prevents hero from leaving tracks
 the game now automatically tracks which sell prices and buy prices you have
         seen for each type of item; these are visible in the discoveries list,