surface: Add A10 Thunderbolt II attack aircraft.

It's almost just a copy of F15 with altered model for the time being.
surface: Add cockpit model to A10.

Also adjusted cockpit position of A10 to match the model.
Make it valid to get null reference in Entity::sq_refobj().
surface: Make A10 to be able to carry various armamanets.

They are currently HydraRocketLauncher, HellfireLauncher and
SidewinderLauncher.  The fire event handler in Squirrel script is
unified to handle all 3 types in a function.