Plane: account for sprung throttle in VTOL throttle suppression

when RC_OPTIONS has been changed to not check throttle for arming then
treat this like a sprung throttle for quadplane throttle suppression
in auto-throttle modes, and only unsuppress when throttle goes above
trim+dz

ArduPlane/quadplane.cpp

@@ -1793,8 +1793,17 @@ void QuadPlane::update_throttle_suppression(void)
         return;
     }
 
-    // if the users throttle is above zero then allow motors to run
-    if (!is_zero(plane.get_throttle_input())) {
+    /* if the users throttle is above zero then allow motors to run
+
+       if the user has unset the "check throttle zero when arming"
+       then the RC controller has a sprung throttle and we should not
+       consider non-zero throttle to mean that pilot is commanding
+       takeoff unless in a manual thottle mode
+    */
+    if (!is_zero(plane.get_throttle_input()) &&
+        (rc().arming_check_throttle() ||
+         plane.control_mode->is_vtol_man_throttle() ||
+         plane.channel_throttle->norm_input_dz() > 0)) {
         return;
     }