Plane: allow control_mode enum to be in arbitrary order

never use control_mode >= BLAH. This requires the order to be important which greatly increases chance for unexpected behavior
Closes https://github.com/ArduPilot/ardupilot/issues/4656

ArduPlane/Plane.h

@@ -561,6 +561,9 @@ private:
     // with STICK_MIXING=0
     bool auto_navigation_mode:1;
     
+    // this allows certain flight modes to mix RC input with throttle depending on airspeed_nudge_cm
+    bool throttle_allows_nudging:1;
+
     // this controls throttle suppression in auto modes
     bool throttle_suppressed;
 	

ArduPlane/navigation.cpp

@@ -135,7 +135,7 @@ void Plane::calc_airspeed_errors()
     }
 
     // Bump up the target airspeed based on throttle nudging
-    if (control_mode >= AUTO && airspeed_nudge_cm > 0) {
+    if (throttle_allows_nudging && airspeed_nudge_cm > 0) {
         target_airspeed_cm += airspeed_nudge_cm;
     }
 

ArduPlane/servos.cpp

@@ -581,7 +581,7 @@ void Plane::set_servos(void)
     // setup flap outputs
     set_servos_flaps();
     
-    if (control_mode >= FLY_BY_WIRE_B ||
+    if (auto_throttle_mode ||
         quadplane.in_assisted_flight() ||
         quadplane.in_vtol_mode()) {
         /* only do throttle slew limiting in modes where throttle

ArduPlane/system.cpp

@@ -327,6 +327,7 @@ void Plane::set_mode(enum FlightMode mode, mode_reason_t reason)
     switch(control_mode)
     {
     case INITIALISING:
+        throttle_allows_nudging = true;
         auto_throttle_mode = true;
         auto_navigation_mode = false;
         break;
@@ -335,17 +336,20 @@ void Plane::set_mode(enum FlightMode mode, mode_reason_t reason)
     case STABILIZE:
     case TRAINING:
     case FLY_BY_WIRE_A:
+        throttle_allows_nudging = false;
         auto_throttle_mode = false;
         auto_navigation_mode = false;
         break;
 
     case AUTOTUNE:
+        throttle_allows_nudging = false;
         auto_throttle_mode = false;
         auto_navigation_mode = false;
         autotune_start();
         break;
 
     case ACRO:
+        throttle_allows_nudging = false;
         auto_throttle_mode = false;
         auto_navigation_mode = false;
         acro_state.locked_roll = false;
@@ -353,6 +357,7 @@ void Plane::set_mode(enum FlightMode mode, mode_reason_t reason)
         break;
         
     case CRUISE:
+        throttle_allows_nudging = false;
         auto_throttle_mode = true;
         auto_navigation_mode = false;
         cruise_state.locked_heading = false;
@@ -365,6 +370,7 @@ void Plane::set_mode(enum FlightMode mode, mode_reason_t reason)
         break;
 
     case FLY_BY_WIRE_B:
+        throttle_allows_nudging = false;
         auto_throttle_mode = true;
         auto_navigation_mode = false;
         
@@ -376,12 +382,14 @@ void Plane::set_mode(enum FlightMode mode, mode_reason_t reason)
 
     case CIRCLE:
         // the altitude to circle at is taken from the current altitude
+        throttle_allows_nudging = false;
         auto_throttle_mode = true;
         auto_navigation_mode = true;
         next_WP_loc.alt = current_loc.alt;
         break;
 
     case AUTO:
+        throttle_allows_nudging = true;
         auto_throttle_mode = true;
         auto_navigation_mode = true;
         if (quadplane.available() && quadplane.enable == 2) {
@@ -397,6 +405,7 @@ void Plane::set_mode(enum FlightMode mode, mode_reason_t reason)
         break;
 
     case RTL:
+        throttle_allows_nudging = true;
         auto_throttle_mode = true;
         auto_navigation_mode = true;
         prev_WP_loc = current_loc;
@@ -404,6 +413,7 @@ void Plane::set_mode(enum FlightMode mode, mode_reason_t reason)
         break;
 
     case LOITER:
+        throttle_allows_nudging = true;
         auto_throttle_mode = true;
         auto_navigation_mode = true;
         do_loiter_at_location();
@@ -418,6 +428,7 @@ void Plane::set_mode(enum FlightMode mode, mode_reason_t reason)
 
     case AVOID_ADSB:
     case GUIDED:
+        throttle_allows_nudging = true;
         auto_throttle_mode = true;
         auto_navigation_mode = true;
         guided_throttle_passthru = false;
@@ -434,6 +445,7 @@ void Plane::set_mode(enum FlightMode mode, mode_reason_t reason)
     case QLOITER:
     case QLAND:
     case QRTL:
+        throttle_allows_nudging = true;
         auto_navigation_mode = false;
         if (!quadplane.init_mode()) {
             control_mode = previous_mode;