Keep the same aircraft on sim reset

When user has installed more aircrafts with the same short ID, preserve
the selected dir, so we use the exact same aircraft.

src/Main/fg_init.cxx

@@ -796,6 +796,13 @@ int fgInitAircraft(bool reinit, bool didUseLauncher)
 
     SGSharedPtr<Root> pkgRoot(globals->packageRoot());
     SGPropertyNode* aircraftProp = fgGetNode("/sim/aircraft", true);
+    SGPropertyNode* aircraftDirProp = fgGetNode("/sim/aircraft-dir", true);
+
+    // ensure aircraft-dir survives reset, so we find the same aircraft
+    // after a reset as we did prior to it. Without this, aircraft
+    // can unintentionally change on a reset
+    aircraftDirProp->setAttribute(SGPropertyNode::PRESERVE, true);
+
     const string fullyQualifiedAircraftId = fgGetString("/sim/aircraft-id");
     string aircraftId = fullyQualifiedAircraftId.empty() ? aircraftProp->getStringValue() : fullyQualifiedAircraftId;
 
@@ -1404,7 +1411,6 @@ void fgStartNewReset()
                "Some errors restoring preserved state (read-only props?)" );
     }
 
-    fgGetNode("/sim")->removeChild("aircraft-dir");
     fgInitAircraftPaths(true);
     fgInitAircraft(true, false /* not from launcher */);
     

src/Main/fg_scene_commands.cxx

@@ -101,6 +101,19 @@ static bool
 do_switch_aircraft (const SGPropertyNode * arg, SGPropertyNode * root)
 {
     fgSetString("/sim/aircraft", arg->getStringValue("aircraft"));
+
+    // we could validate the new aircraft is find-able, and if not, bail out
+    // of the command at this point? Might give a better user experience?
+    // would need something like bool fgIsValidAircraft(name, dir);
+    // which could be cooked up by modifying FindAndCacheAircraft
+
+    // We assume that we are changing the aircraft, so we remove the aircraft path.
+    // this forces aircraft-dir to be recomputed by a full search as the user hopefully
+    // expects. 
+    // We could allow pasing the dir as an optional argument to this command, if that
+    // is ever needed in the future.
+    fgGetNode("/sim")->removeChild("aircraft-dir");
+
     // start a reset
     fgResetIdleState();
     return true;