Start of an internal 3-D view. If the property "/cockpit" (temporary

Start of an internal 3-D view.  If the property "/cockpit" (temporary
and soon to be renamed) is true, FlightGear will draw the 3-D model
even in internal view.  This makes sense right now only with the C310
model, since the others don't have any kind of interiors modeled and
all surfaces in all models are one-sided.

This isn't quite working yet -- the view code is very hard to
disentangle, and currently, if the view is not forward, roll and pitch
are applied incorrectly.  It looks interesting (modulo a messy model)
on the ground, in level flight, or looking straight ahead under any
other flight conditions.