Freeciv

Noticed while testing #629 on main; probably already present in older versions if they already have requirement types with the appropriate behavior.

When a player does not have enough information to know whether a build requirement is satisfied, the city dialog might not show them that they can build it, but if they try anyway, the server will tell them whether the city can build it (by either building or postponing/purging it). Maybe cities should be unable to build something unless their owner knows for sure they can build it? But that might be over-restrictive, especially given that not all requirement types are fully metaknowledge-supported.