Fixed plane behavior issues from http://springrts.com/mantis/view.php?id=4541
 * Planes turn at the correct distance from their move goal.
 * Planes no longer become confused by terrain changes.
 * Fixed units circling around move goal instead of strafing through.
 * Fixed occasional roll jittering which occurs just after passing a move goal.
Merge pull request #129 from GoogleFrog/PlaneBehaviourFix

fix #4541: Fixed plane behavior issues