Special mushrooms no longer trivially identifiable

Fixes #2853.

Makes poison and magic mushrooms have identical nutrition/fun stats as the base mushroom, so they can't be identified by just being examined. Instead, Survival 3 will identify poison ones, and Survival 5 will identify magic ones in the descriptions when you examine them. (Comments appreciated on skills/levels for that)

They still won't stack with each other, but there's nothing I can do about that. At least now you don't know what the stacks are without eating one.

The poison effect is unchanged; for magic mushrooms, it still adds the morale bonus in a new iuse function that basically does the same thing as before.