Improve handling of the pushBackRoute attribute in groundnet's Parking nodes

- Don't ignore pushbackRoute="0".

- Stricter parsing with precise log messages when the input is
  incorrect.

- Add missing includes in src/Airports/dynamicloader.cxx.

See <https://sourceforge.net/p/flightgear/mailman/message/35788373/> for
the discussion about this change.

src/Airports/dynamicloader.cxx

@@ -17,6 +17,7 @@
 #  include <config.h>
 #endif
 
+#include <string>
 #include <cstdlib>
 #include <cstring> // for strcmp
 
@@ -26,6 +27,9 @@
 #include <Airports/airport.hxx>
 #include <Airports/dynamics.hxx>
 #include <Airports/groundnetwork.hxx>
+#include <simgear/misc/strutils.hxx>
+#include <simgear/debug/logstream.hxx>
+#include <simgear/structure/exception.hxx>
 
 using std::string;
 
@@ -92,7 +96,9 @@ void FGGroundNetXMLLoader::startParking(const XMLAttributes &atts)
   double heading = 0.0;
   double radius = 1.0;
   string airlineCodes;
-  int pushBackRoute = 0;
+  int pushBackRoute = -1;       // signals unseen attribute
+
+  savePosition();               // allows to retrieve the line number
 
   for (int i = 0; i < atts.size(); i++)
   {
@@ -120,7 +126,15 @@ void FGGroundNetXMLLoader::startParking(const XMLAttributes &atts)
 	  else if (attname == "airlineCodes")
       airlineCodes = atts.getValue(i);
     else if (attname == "pushBackRoute") {
-      pushBackRoute = std::atoi(atts.getValue(i));
+      const string attrVal = atts.getValue(i);
+      try {
+        pushBackRoute = simgear::strutils::readNonNegativeInt<int>(attrVal);
+      } catch (const sg_exception& e) {
+        SG_LOG(SG_NAVAID, SG_DEV_WARN,
+               getPath() << ":" << getLine() << ": " <<
+               "invalid value for 'pushBackRoute': " << e.what());
+        pushBackRoute = -2;
+      }
     }
   }
 
@@ -130,7 +144,7 @@ void FGGroundNetXMLLoader::startParking(const XMLAttributes &atts)
                                      pos, heading, radius,
                                      gateName + gateNumber,
                                      type, airlineCodes));
-  if (pushBackRoute > 0) {
+  if (pushBackRoute >= 0) {
     _parkingPushbacks[parking] = pushBackRoute;
   }