Previously pilot models would stay visible when the
walker was active. Which is a bit unrealistic given
the walker is supposed to be "you" in some way.
So now the internal pilot is hidden when the walker
is active outside the aircraft.
Fix provided by Benedikt Hallinger
Adds standard view option to enable pilot models.
Set /sim/rendering/pilot-model/available=true in aircraft
using pilot models.
/sim/rendering/pilot-model/enabled is used by users to
enable/disable the pilot models from the View Options dialog.
Note that pilot models are only visible for non-internal views.
