(svn r2288) - CodeChange: protected the next batch of commands (41 so far, out of 115).

- CodeChange: changed the airport gui airport-type checking. Added function GetValidAirports() that returns bitmasked availibility, is also used for checking.
- CodeChange: to check tree-planting, 2 const arrays have been moved to table/tree_land.h (type and count)
- CodeChange: added IsTownIndex() in following of IsStationIndex(), etc.
- Fix (regression): road tunnels did not work anymore, forgot that their type was 0x200 (documented now)

airport.h

@@ -48,4 +48,20 @@ void InitializeAirports(void);
 void UnInitializeAirports(void);
 const AirportFTAClass* GetAirport(const byte airport_type);
 
+/** Get buildable airport bitmask.
+ * @return get all buildable airports at this given time, bitmasked.
+ * Bit 0 means the small airport is buildable, etc.
+ * @todo set availability of airports by year, instead of airplane
+ */
+static inline uint32 GetValidAirports(void)
+{
+	uint32 bytemask = _avail_aircraft; /// sets the first 3 bytes, 0 - 2, @see AdjustAvailAircraft()
+
+	// 1980-1-1 is --> 21915
+	// 1990-1-1 is --> 25568
+	if (_date >= 21915) {SETBIT(bytemask, 3);}	// metropilitan airport 1980
+	if (_date >= 25568) {SETBIT(bytemask, 4);}	// international airport 1990
+	return bytemask;
+}
+
 #endif /* AIRPORT_H */