Remove some unnecessary checks for state=menu
Add disease resistance, with corresponding shield status and spell.
- Infected altar now inflicts disease shield in addition to stoneskin in the good outcome
- Infect enchantment boosts disease resistance instead of poison resistance on defensive items
- Disease resistance added to many monsters, especially those with disease attacks
- Undead are immune to disease, except spirits

Non-disease-related tweaks:
- Claximander gains shock resistance
- Acid burn is adjusted so that those with corrosion vulnerability are burned more quickly
Slightly better handling of status values that are not initialized
Tweak some shield messages
Framework for augmentations received on level-up; also gaining a spell from this is now restricted to mages.