Make target-city-must-be-known precise
The rule that a city must be known to a player before he can perform an action to it used to be content when the player knew the city's tile. It was therefore possible to act against an unknown city founded at a tile observed before the city was founded.
Add player_tile_city_id_get() to the Freeciv interface. This makes it possible to check if the city it self is known. Use it to make the rule precise.