AC_WPNav: smooth waypoint by freezing feed-forward and allowing overshoot

First part of this fix is freezing the position controller's xy-axis
feed foward as we transition to the new segment.
Second part is work-around for straight line segments in that we allow
the target point to actually overshoot the end of the segment by up to
2m if the segment is a "fast waypoint".  Ideally we would instead notice
the waypoint has been completed and take any left over time or distance
and move our target along the track towards the next waypoint but that
would require a much larger change to allow the wpnav lib to hold the
next two waypoints.