Allow pStun to be generated on non-body armor randarts.

This is a fix such that pStun can actually be generated on randarts which are not body armor.

It's actually Magnate who's provided the fix (which the commit message should hopefully say). 

See also http://angband.oook.cz/forum/showthread.php?t=6152

EDIT: Add a commit to use N_ELEMENTS instead of hardcoded constants