Improvements and supplements to PoseDir

[DeathFishAtEase]

#1149 introduced customization of aircraft landing direction, but broke some vanilla behaviors. This PR restores them and adds toggles / separate definitions.

• Handle the differences caused by Phobos calculation issues when aircraft land on open terrain in mods, and leave separate definitions
• Add a toggle to deprecate handling of airport building direction for landing direction
• Supplement documentation for the breaking changes introduced by PR#1149.

[github-actions]

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