GCS_MAVLink: generated files for new geofence SYS_STATUS & CMD messages
Plane: now sending MAV_SYS_STATUS_GEOFENCE status.
Plane: Now maintaining previous control mode in a global state variable.
Plane: Fixed bug.  Now goes to previous control mode when leaving GUIDED

    Also laying foundations for automatic and GCS enabled fences:
        Geofence now knows when PWM caused the fence to enable.
Plane: Fence changes to allow for auto and gcs enabled fences.
Plane: Fixed build errors when #define GEOFENCE_ENABLED != ENABLED
Plane: FENCE_RET_RALLY param. Plane returns to rally point on breach.
Plane: autofence changes needed after AP_Mission arrived.