bus: beef up parameter checking in vtable handling
bus: make sure introspection can be generated correctly with NULL method signatures
bus: add sd_bus_message_append_strv() to easily add a NULL terminated string array to a message
bus: properly generate GetAll reply
bus: unify code to find parents of an object path with
bus: in GetManagedObjects() only return each object once.
bus: properly implement logic for generating InterfacesAdded/InterfacesRemoved signals of ObjectManager