io: Rework default callbacks

Register a dummy callback struct for default callbacks. Handle them in a
separate function which will later allow to return meaningful error
codes.

python/generator.py

@@ -364,6 +364,7 @@ deprecated_funcs = {
     'xmlRecoverDoc': True,
     'xmlRecoverFile': True,
     'xmlRecoverMemory': True,
+    'xmlRegisterHTTPPostCallbacks': True,
     'xmlRelaxNGCleanupTypes': True,
     'xmlRelaxNGInitTypes': True,
     'xmlRemoveRef': True,