AP_Common: spline flag added to Location structure

Ideally this flag should be part of the AP_Mission command structure
Mission: populate Location spline flag based on param3

To-Do: add MAVlink support to somehow indicate a mission waypoint should
be flown to use a spline or straight path
Mission: minor comment update
WP_Nav: first draft of spline support
Copter: integrate first draft of spline support