raise signal "/sim/signals/nasal-dir-initialized" when all Nasal files

in $FG_ROOT/Nasal/* were loaded and executed, and thus all Nasal library
functions are available. This was in the past only done with settimer(..., 0),
constructions, but suchlike triggered timer functions are executed
*after* aircraft specific Nasal files were loaded, so they can't be
used for internal library initialization, where e.g. props.Node() is
already needed.

src/Scripting/NasalSys.cxx

@@ -426,6 +426,7 @@ void FGNasalSys::init()
         loadModule(fullpath, file.base().c_str());
     }
     ulCloseDir(dir);
+    fgSetBool("/sim/signals/nasal-dir-initialized", true);
 
     // Pull scripts out of the property tree, too
     loadPropertyScripts();