New body armor skill and shield skill

Body armor and shields now grant bonus AC and MC based on weight, but
capped by their respective skills.  This should make heavier body armor
less bad compared to mithril coats and dragon scale mails, and shields
less bad compared to two-weaponing, two-handed weapons and spellcasting.
Wearing heavier body armor and heavier shields speeds up training.  Read
the changelog for full details.

Due to how the shield skill works, the shield of reflection now weighs
50 (like in NetHack) instead of 75 (like in UnNetHack).

Breaks save compatibility.