(Ego) weapons should not be immediately destroyed by acid

[Pete Mack]

From email list:
Can I [UnAndrew] suggest changing item damage to make wearable items in the backpack not be instantly destroyed if they get damage? Codes already
in Unangband, and it makes it more worthwhile having swap armour and
weapons.

[Pete Mack] Ego armor is generally either acid-proof or not worth swapping, but it's a good idea for weapons. Currently swap weapons just don't work very well.

[Other Acid damage stuff deleted for now]