NavFunctions: Use precise formula in FindLatitudeLongitude()

The previously used formula is an approximation for dlon < pi/2. Tests have
shown though that this approximation is returning much larger errors than
acceptable.

Example: FindLatitudeLongitude(51 N 7 W, 135 deg, 50 km) returns a point
that is about 50.186 km away. Using the more precise formula fixes this
problem and returns a point that is exactly 50.000 km away.

This is relevant for the AirspaceParser that uses this function to convert
arcs into polygons.

see http://williams.best.vwh.net/avform.htm#LL