Aircraft could fall through the ground on scenery reload. Properly reset
the "sceneryloaded" flag to pause FDM processing while scenery is being
reloaded. Also stop processing some instruments while scenery is reloaded.
This allows to display objects that are in /ai/models/, but not managed
by the AI manager, and it follows fgfs' design principle that subsystems
should communicate over the property tree (if possible). This is a tad
slower, but the radar is only updated once every second.
