AP_Navigation: added a turn_distance() method with turn_angle
AP_L1_Control: implement turn_distance() with turn angle

uses a linear approximation for now.
Plane: calculate next_turn_angle to prevent early WP completion

this should help for ball drop and camera trigger with straight line
runs
GCS_MAVLink: added MAV_CMD_DO_INVERTED_FLIGHT

used to invert from a mission
GCS_MAVLink: rebuild MAVLink headers
AP_Mission: added support for MAV_CMD_DO_INVERTED_FLIGHT
Plane: added MAV_CMD_DO_INVERTED_FLIGHT support

allows for mission control over inverted flight
Spline div zero and leash limit fix
AC_WPNav: spline div by zero fix

Also add check for straight line navigation to ensure speed is not
reduced below zero when it hits the leash limit
Also minor formatting changes