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
AutoTest: fix error reporting for copter AVC mission
Copter: Improve surface tracking dynamics

This little fix will allow much higher sonar_gain without the
oscillating effect it used to bring.