Convert FGNavRecord to inherit FGPositioned. This is much more self-contained than the FGRunway change, since FGNavRecord already had good encapsulation of its state. However, it's a large diff due to moving around two nasty pieces of code - the 'align navaid with extended runway centerline' logic and the 'penalise transmitters at the opposite runway end' logic.
In general things are more readable because I've replaced the Navaid type enum, and the use of Robin's integer type codes, with switches on the FGPositioned::Type code - no more trying to recall that '6' is an outer marker in Robin's data. The creation code path is also pushed down from navdb into navrecord itself.
- this exposed a bizarre issue on Mac where dragging in <AGL/agl.h> in
extensions.hxx was pulling in all of Carbon to the global namespace
- very scary. As a result, I now need to explicitly include CoreFoundation
in fg_init.cxx.
- change SG_USING_STD(x) to using std::x
