Setting a parking position (e.g. by default via command-line) caused
"Position aircraft in air" to not work as expected. The "parkpos" property
must be reset, otherwise heading/glidepath/azimuth do not have any effect
when repositioning.
Make a new set location (on ground) dialog box which knows how to set up
the /sim/presets/ properties correctly for a new ground position.
For now leave the previous all-in-one dialog box for in-air resets. However
this really needs some more work to make sure the /sim/presets properties get
set correctly.
