Fix for own recent reference count error.

- The armature weakref list was  being incref'd twice then decrefed twice (incref and decref were used incorrectly), now only once. My 'fix' broke this.
- In bpy_pydriver_create_dict the 2 refs added from running PyDict_SetItemString twice were undone when clearing the dictionary (added comment)
- changed Py_XDECREF to Py_DECREF int BPY_pyconstraint_update and BPY_pyconstraint_target, Py_XDECREF checs for NULL value which would have crashed blender before it got to Py_XDECREF anyway.
- after every error is reported (PyErr_Print), remove sys.last_traceback and clear the error, I found this fixed certain crashes (usually when starting the game engine or exiting blender), so best do this all the time.

- header_text.c, CcdPhysicsEnvironment.cpp, KX_CameraActuator.cpp - remove some warnings.

source/gameengine/GameLogic/SCA_PythonController.cpp

@@ -300,7 +300,7 @@ bool SCA_PythonController::Compile()
 		 * their user count. Not to mention holding references to wrapped data.
 		 * This is especially bad when the PyObject for the wrapped data is free'd, after blender 
 		 * has alredy dealocated the pointer */
-		PySys_SetObject( (char *)"last_traceback", Py_None);
+		PySys_SetObject( (char *)"last_traceback", NULL);
 		PyErr_Clear(); /* just to be sure */
 		
 		return false;
@@ -358,7 +358,7 @@ void SCA_PythonController::Trigger(SCA_LogicManager* logicmgr)
 		 * their user count. Not to mention holding references to wrapped data.
 		 * This is especially bad when the PyObject for the wrapped data is free'd, after blender 
 		 * has alredy dealocated the pointer */
-		PySys_SetObject( (char *)"last_traceback", Py_None);
+		PySys_SetObject( (char *)"last_traceback", NULL);
 		PyErr_Clear(); /* just to be sure */
 		
 		//PyRun_SimpleString(m_scriptText.Ptr());