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
Merge branch 'master' into demo-staging