ArduSub: move orderly rebooting code from GCS into AP_Vehicle

Several places we reboot the vehicle we should probably do several of
the things done in this code - flushing parameters, forcing safety on
etc.

ArduSub/GCS_Mavlink.h

@@ -18,8 +18,6 @@ protected:
 
     uint8_t sysid_my_gcs() const override;
 
-    bool should_zero_rc_outputs_on_reboot() const override { return true; }
-
     MAV_RESULT handle_command_do_set_roi(const Location &roi_loc) override;
     MAV_RESULT _handle_command_preflight_calibration_baro() override;
     MAV_RESULT _handle_command_preflight_calibration(const mavlink_command_long_t &packet) override;

ArduSub/Sub.h

@@ -128,6 +128,10 @@ public:
 
     Sub(void);
 
+protected:
+
+    bool should_zero_rc_outputs_on_reboot() const override { return true; }
+
 private:
 
     // key aircraft parameters passed to multiple libraries