Reset: ensure mobile tacan drop its AI references

Add a slightly ugly hook, to force mobile nav records to drop their
refs into the property tree on reset.

src/Main/fg_init.cxx

@@ -1269,7 +1269,10 @@ void fgStartNewReset()
             // don't delete here, dropping the ref should be sufficient
         }
     } // of top-level groups iteration
-    
+
+    // drop any references to AI objects with TACAN
+    flightgear::NavDataCache::instance()->clearDynamicPositioneds();
+
     FGRenderer* render = globals->get_renderer();
     // needed or we crash in multi-threaded OSG mode
     render->getViewer()->stopThreading();

src/Navaids/NavDataCache.cxx

@@ -1725,6 +1725,16 @@ void NavDataCache::abortTransaction()
   d->transactionAborted = true;
 }
 
+void NavDataCache::clearDynamicPositioneds()
+{
+    std::for_each(d->cache.begin(), d->cache.end(), [](PositionedCache::value_type& v) {
+        if (v.second->type() == FGPositioned::MOBILE_TACAN) {
+            auto mobile = fgpositioned_cast<FGMobileNavRecord>(v.second);
+            mobile->clearVehicle();
+        }
+    });
+}
+
 FGPositionedRef NavDataCache::loadById(PositionedID rowid)
 {
   if (rowid < 1) {

src/Navaids/NavDataCache.hxx

@@ -336,6 +336,9 @@ public:
         class ThreadedGUISearchPrivate;
         std::unique_ptr<ThreadedGUISearchPrivate> d;
     };
+
+    void clearDynamicPositioneds();
+
 private:
   NavDataCache();
 

src/Navaids/navrecord.cxx

@@ -166,6 +166,11 @@ const SGVec3d& FGMobileNavRecord::cart() const
 }
 
 //------------------------------------------------------------------------------
+void FGMobileNavRecord::clearVehicle()
+{
+    _vehicle_node.clear();
+}
+
 void FGMobileNavRecord::updateVehicle()
 {
   _vehicle_node.clear();

src/Navaids/navrecord.hxx

@@ -126,6 +126,11 @@ class FGMobileNavRecord:
   public FGNavRecord
 {
   public:
+      static bool isType(FGPositioned::Type ty)
+      {
+          return (ty == MOBILE_TACAN);
+      }
+
     FGMobileNavRecord( PositionedID aGuid,
                        Type type,
                        const std::string& ident,
@@ -142,7 +147,9 @@ class FGMobileNavRecord:
     void updateVehicle();
     void updatePos();
 
-  protected:
+    void clearVehicle();
+
+protected:
     SGTimeStamp _last_vehicle_update;
     SGPropertyNode_ptr _vehicle_node;
     double _initial_elevation_ft; // Elevation as given in the config file