Fix crash on —show-aircraft

Change how we return, so QApplication doesn’t explode. exit() is not
a happy thing for QApplication.

Ticket:
https://sourceforge.net/p/flightgear/codetickets/2139/

src/Main/fg_init.cxx

@@ -717,7 +717,9 @@ void fgInitAircraftPaths(bool reinit)
 int fgInitAircraft(bool reinit)
 {
     if (!reinit) {
-        flightgear::Options::sharedInstance()->initAircraft();
+        auto r = flightgear::Options::sharedInstance()->initAircraft();
+        if (r == flightgear::FG_OPTIONS_SHOW_AIRCRAFT)
+            return r;
     }
     
     FindAndCacheAircraft f(globals->get_props());

src/Main/main.cxx

@@ -647,7 +647,9 @@ int fgMainInit( int argc, char **argv )
     configResult = fgInitAircraft(false);
     if (configResult == flightgear::FG_OPTIONS_ERROR) {
         return EXIT_FAILURE;
-    } else if (configResult == flightgear::FG_OPTIONS_EXIT) {
+    } else if ((configResult == flightgear::FG_OPTIONS_EXIT) ||
+               (configResult == flightgear::FG_OPTIONS_SHOW_AIRCRAFT))
+    {
         return EXIT_SUCCESS;
     }
 

src/Main/options.cxx

@@ -2211,7 +2211,7 @@ void Options::initPaths()
 
 }
 
-void Options::initAircraft()
+OptionResult Options::initAircraft()
 {
   string aircraft;
   if (isOptionSet("aircraft")) {
@@ -2246,7 +2246,8 @@ void Options::initAircraft()
                        aircraft_paths.end());
 
     fgShowAircraft(path_list);
-    exit(0);
+    // this is to indicate that we did show it
+    return FG_OPTIONS_SHOW_AIRCRAFT;
   }
 
   if (isOptionSet("aircraft-dir")) {
@@ -2268,6 +2269,8 @@ void Options::initAircraft()
         // can't validate this until the -set.xml is parsed
         fgSetString("/sim/aircraft-state", stateName);
     }
+    
+    return FG_OPTIONS_OK;
 }
 
 void Options::processArgResult(int result)

src/Main/options.hxx

@@ -130,7 +130,7 @@ public:
   /**
    * init the aircraft options
    */
-  void initAircraft();
+  OptionResult initAircraft();
   
   /**
    * should defualt configuration files be loaded and processed or not?