Fix AGL altitude calculation for fixed landing gears and reorganize t…

[jmbau]

…his code a little bit.

AGL altitude calculation when landing is actually wrong for aircrafts with fixed landing gear: it's visible because when a such helicopter lands, AGL altitude gradually decreases, then suddenly drops to zero. This fix it.

Remarks:

• In v1 distance calculation, I added gear_state == 2 which means landing gear "down/deployed/out and fixed". This gear state was already defined in SARSimOpLandingGear(...) in simutils.c, lines 917 to 923.
• Of course instead of (gear_state == 1) || (gear_state == 2) it is possible to write gear_state >= 1 but I think that it is less clear, and more risky if a new gear state is added later.
• Regarding code reorganization, I think that it makes more sense now: first, calculate the value, then, convert it to the right unit.
• At take off, whatever landing gear type, AGL altitude suddently jumps from zero to 0.3m (~ 1.0ft): this is because of sfmsimforce.c, line 1635.