(svn r6770) -Codechange: Make the airport checking a bit stricter...fix unnoticed errors:
Commuter (20): invalid use of terminal group designator; since all other options
goto position 2 and no additional blocks are checked, simplify state
Intercontinental (23, 30): no extra states are needed since all options go to
position 70 and 31 unconditionally
Intercontinental (31): invalid use of more-options-to-follow (255)

(svn r6769) -Codechange: Minor syntax, const correctness, variable localization, coding
style changes to airport code, and cleanup of airport.c. Should not change
any functionality.

(svn r6768) -Cleanup: For airports change *Airport to *apc (variable naming coding style
conformance (AirportFTAClass), *FA to *apFA (to better reflect its type
AirportFTAbuildup), and ->next_in_chain into ->next.

(svn r5346) - Feature: Add 4 new airports. 2 for aircraft, 2 for helicopters.
Commuter airport: Small. 5x4. 3 terminals, 2 helipads.
Intercontinental: massive. 9x11. 8 terminals, 2 helipads, 4 runways.
Helidepot: a small heliport with a depot for helis only.
Helistation: a large heliport with 3 helipads and a depot.

(svn r1440) -Feature: Allows more flexible airport layouts now, as the number of
terminals in every group can be freely chosen
-Codechange: AT_OILRIG is now 15, so that new airports can be added
easily.