Regularized the position initialization:

Regularized the position initialization:

1. If /position/longitude-deg and /position/latitude-deg are in range,
   use them.
2. Otherwise, if /sim/startup/airport-id is not empty, use it.
3. Otherwise, set the lon/lat to the middle of the KSFO field.

The default used to be Globe, AZ, but that doesn't make sense since we
don't distribute that scenery by default any more.

With this change, starting from a save file seems to work properly:

  fgfs myflight.sav