bundle2: declare supported parameters for all handlers We now update all existing handlers with the supported parameters information.

bundle2: make it possible to declare params handled by a part handler If we are to enforce the mandatory aspect of parameter, we need a way to discover what a handler supports. The best option we end up with is this a simple declaration of known parameters at registration time. We simply plug the list of parameters on the function object because Python lets us do that and there is no benefit for a more complicated way. One of the handlers is updated for example and testing.

bundle2: support transmission of params error over the wire We picked a null character to split each parameter during the transfer. This is fragile if the same character is used in parameter name. However other codes will already behave in a strange way in that case, so we are not introducing any regression. A better format may be picked for the final version of the protocol.