Neighbors determine valid FSes to adopt before attempting to adopt one.

Neighbors determine valid FSes to adopt before attempting to adopt one, and adopt only valid FSes. Valid FSes are those allowed by the game rules, not already taken, not conflicting with any already taken, and, if the arcology is ruled a government loyal to the player, not conflicting with the player arcology.

Fixes #2404 (closed), and cuts neighbor adoption code by a few hundred lines without losing any text. Also fixes a few bugs (perhaps most importantly, adopting Neo-Imperialism as your only FS will no longer break the rival arcology).