Add fgInitNav() function so that navaids and fixes can be loaded

Add fgInitNav() function so that navaids and fixes can be loaded
earlier, before fgInitSubsystems().

Modify fgInitPos() so that the plane is not automatically aligned with
a runway when an airport is the reference point unless (a) a runway
was explicitly requested, or (b) the plane is on the ground with no
offset distance specified.  To set up the plane lined up on an
approach to a runway, use something like

  fgfs --airport=CYOW --runway=32 --altitude=300 --offset-distance=0.5

This way, it's possible to specify a starting position relative to an
airport without getting snapped onto a runway approach (unless you
want to be).