surface: Trying to make autopilot approach logic scripted like landing.

Rudder target is added to returned values of aerialLanding function.

Now we can rapidly prototype approaching autopilot with the method used
in landing AI.

Also a bug is fixed in Entity::sq_pushobj() that could crash the whole
simulation by NULL pointer reference.
Fix lacking const qualifier in SQIntrinsic constructor argument.
Revived Entity getters and setters for intrinsic types from Squirrel.

For details and rationale, refer to the comments added to Entity.cpp in
this commit.
Add estimate_pos implementation into aerialLanding().