Call fgInitAllowedPaths() also in fgInitConfig()

This is an additional call to fgInitAllowedPaths(), earlier than the
normal one. It doesn't know the paths given to --allow-nasal-read, but
can already authorize read access for all of $FG_ROOT. This allows one
to add SGPath::validate() checks for paths given to
ResourceManager::findPath() with a non-null second argument (without
this change, the validation would fail for files included from
defaults.xml, read by fgSetDefaults() before the definitive
fgInitAllowedPaths() call has been performed).

src/Main/fg_init.cxx

@@ -711,6 +711,7 @@ int fgInitConfig ( int argc, char **argv, bool reinit )
         if (result != flightgear::FG_OPTIONS_OK) {
             return result;
         }
+        fgInitAllowedPaths();   // preliminary: whitelist at least FGData
     }
 
     // establish default for developer-mode based upon compiled build types