Plane: added THR_FAILSAFE=2 option

this allows for RC inputs to be ignored at low throttle, but won't
trigger failsafe. It is meant for users flying BVLOS missions, where
they want GCS failsafe enabled, but don't want RC failsafe, and want
to be sure that RC inputs will be ignored at low RC throttle values

Thanks to suggestion from Pompecukor

ArduPlane/AP_Arming.cpp

@@ -58,7 +58,7 @@ bool AP_Arming_Plane::pre_arm_checks(bool display_failure)
     }
 
     if (plane.channel_throttle->get_reverse() && 
-        plane.g.throttle_fs_enabled &&
+        Plane::ThrFailsafe(plane.g.throttle_fs_enabled.get()) != Plane::ThrFailsafe::Disabled &&
         plane.g.throttle_fs_value < 
         plane.channel_throttle->get_radio_max()) {
         check_failed(display_failure, "Invalid THR_FS_VALUE for rev throttle");

ArduPlane/Parameters.cpp

@@ -460,10 +460,10 @@ const AP_Param::Info Plane::var_info[] = {
 
     // @Param: THR_FAILSAFE
     // @DisplayName: Throttle and RC Failsafe Enable
-    // @Description: This enables failsafe on loss of RC input. How this is detected depends on the type of RC receiver being used. For older radios an input below the THR_FS_VALUE is used to trigger failsafe. For newer radios the failsafe trigger is part of the protocol between the autopilot and receiver.
-    // @Values: 0:Disabled,1:Enabled
+    // @Description: This enables failsafe on loss of RC input. How this is detected depends on the type of RC receiver being used. For older radios an input below the THR_FS_VALUE is used to trigger failsafe. For newer radios the failsafe trigger is part of the protocol between the autopilot and receiver. A value of 2 means that the RC input won't be used when throttle goes below the THR_FS_VALUE, but it won't trigger a failsafe
+    // @Values: 0:Disabled,1:Enabled,2:EnabledNoFailsafe
     // @User: Standard
-    GSCALAR(throttle_fs_enabled,    "THR_FAILSAFE",   1),
+    GSCALAR(throttle_fs_enabled,    "THR_FAILSAFE",   int(ThrFailsafe::Enabled)),
 
 
     // @Param: THR_FS_VALUE

ArduPlane/Plane.h

@@ -1118,6 +1118,12 @@ private:
         CROW_DISABLED,
     };
 
+    enum class ThrFailsafe {
+        Disabled    = 0,
+        Enabled     = 1,
+        EnabledNoFS = 2
+    };
+
     CrowMode crow_mode = CrowMode::NORMAL;
 
 public:

ArduPlane/radio.cpp

@@ -285,7 +285,7 @@ void Plane::control_failsafe()
         }
     }
 
-    if(g.throttle_fs_enabled == 0) {
+    if (ThrFailsafe(g.throttle_fs_enabled.get()) != ThrFailsafe::Enabled) {
         return;
     }
 
@@ -378,7 +378,7 @@ bool Plane::trim_radio()
  */
 bool Plane::rc_throttle_value_ok(void) const
 {
-    if (!g.throttle_fs_enabled) {
+    if (ThrFailsafe(g.throttle_fs_enabled.get()) == ThrFailsafe::Disabled) {
         return true;
     }
     if (channel_throttle->get_reverse()) {