Denote unusable or restricted slots in the mutation list

[PowerOfKaishin]

Just tried to put a helmet on a Draconian after getting rid of a beak mutation with another !mut. It seems obvious that it still shouldn't work with those horns, but bear in mind that Gargoyles can wear armour despite giant wings, so I think this needs clarification in the mutation list.

[Terrapin]

Imo, because this is already shown on the % screen, there's no need to have it under mutations as well. Then again, why not?