[Ready] Enables sundeath

Deals mortal damage to SUNDEATH-having critters when they're in the sun, gives Nifty Flavor Text for that and the regenerators so you know what's going on, and makes SUNDEATH critters also avoid the sun.

(Testing it was fun: spawn dark wyrms around the shelter.  Especially when I spawned one close enough to get inside in time...and chase me through the shelter to the other side.  Whereupon it smashed the doors and patrolled the door.

Addresses #4604 .

Turned out the Shadow issue was confusion over their self-defeating Special.  :-/  Merge when ready, please.