MP startup: exclude parking positions as hold-shorts

Only consider actual taxi nodes, not parking positions, as potential
hold-short locations, when doing an MP runway-start relocation. This
avoids some odd behaviours for groundnets which only define
parking positions and nothing else.

src/Airports/groundnetwork.cxx

@@ -234,6 +234,13 @@ FGTaxiNodeRef FGGroundNetwork::findNearestNodeOffRunway(const SGGeod& aGeod, FGR
                  [runwayLine, cartPos, marginMSqr] (const FGTaxiNodeRef& a)
     {
         if (a->getIsOnRunway()) return false;
+        
+        // exclude parking positions from consideration. This helps to
+        // exclude airports whose ground nets only list parking positions,
+        // since these typically produce bad results. See discussion in
+        // https://sourceforge.net/p/flightgear/codetickets/2110/
+        if (a->type() == FGPositioned::PARKING) return false;
+        
         return (distSqr(runwayLine, a->cart()) >= marginMSqr);
     });