Respect different behaviour of RS and SR flip flops

No more invalid states here.
According to IEC 61131, RS flip flop have dominant reset and SR have
dominant set. This is now implemented.

src/Autopilot/xmlauto.cxx

@@ -918,9 +918,15 @@ void FGXMLAutoLogic::update(double dt)
 }
 
 class FGXMLAutoRSFlipFlop : public FGXMLAutoFlipFlop {
+private:
+  bool _rs;
 public:
   FGXMLAutoRSFlipFlop( SGPropertyNode * node ) :
-    FGXMLAutoFlipFlop( node ) {}
+    FGXMLAutoFlipFlop( node ) {
+      // type exists here, otherwise we were not constructed
+      string val = node->getNode( "type" )->getStringValue();
+      _rs = (val == "RS");
+    }
 
   void updateState( double dt ) {
 
@@ -940,9 +946,13 @@ public:
     // s == false && q == false: no change, keep state
     if( s || r ) {
       bool q = false;
+      if( _rs ) { // RS: reset is dominant
         if( s ) q = true; // set
         if( r ) q = false; // reset
-      // s && q: race condition. we let r win
+      } else { // SR: set is dominant
+        if( r ) q = false; // reset
+        if( s ) q = true; // set
+      }
       if( inverted ) q = !q;
 
       if ( debug ) cout << "Updating " << get_name() << ":"