Freeciv

Sub-task of #42501 but has a separate meaning, in commercial games we can get territory map from a hut (see http://forum.freeciv.org/f/viewtopic.php?f=13&t=91188).

Common API: int (Unit_Type).vision_radius_sq

Client API: bool (Tile).is_seen (known can be checked by .terrain ~= nil but may et a special method)

Server API:

• bool (Player):can_see(Tile tile, string vlayer = NULL) -- normal vision by default, may supply "Subsurface" or "Stealth"
• bool (Player):can_see(Unit unit) -- Maybe these two can_see() methods should go to common code but in clients they will lie a lot
• bool (Player):knows_map(Tile tile)
• void (Player):give_map_radius_sq(Tile tile, int radius = 1)
• void (Player):give_map(Tile tile, int dist = 1)
• void (Player):forget_map_radius_sq(Tile tile, int radius = 1)
• void (Player):forget_map(Tile tile, int dist = 1)