surface: Add landingAirport to Aerial for specifying location to land to.
surface: Trying to make autopilot be able to land in vain.
surface: Brushed up auto landing AI to be more precise on heading.

It still cannot align pitch precisely to land, but it looks much
nicer than before.
Add means to re-program AI logic on runtime with Squirrel script.
Make roll and throttle be under AI script's control.
The Aerial AI is getting better at aiming to landing position.
surface: Even more improved Aerial autopilot for landing.

Now it can land to an Airport precisely, given that the starting position
is good for landing.

Added aerialLandingSpoiler function to return desired spoiler status
from the autopilot script.

Also added sqGet entries to Airport for querying ILS signals from a
Squirrel script.

The scripted autopilot has proven itself useful for rapid prototyping with
its ability to reload on the fly (during the game is running).
We can run the extremely fast 2-step cycle, that is, tweaking the autopilot
by modifying the script and reloading in the game, to brush up the logic.
surface: Merged multiple autopilot functions in the Squirrel script.

This change is expected to enhance performance a little by reducing
duplicate calculations.
surface: Add functions to simplify codes in Aerial::animAI().

The sqGetter and sqBoolGetter functions reduces duplicate codes and
supplies appropriate exception generations.
surface: Add gear setting for aerialLanding autopilot control.
surface: Get rid of airport landing offset guessing in autopilot.

The delta position without landing site offset yields good results;
why bother calculating complicated things?