When the user provides a function as last argument for socket.emit,
socket.io will flag this as an RPC and add a cb as the last argument
to the client.on('event', ...) handler on the server side.
Without a function as last argument for socket.emit, the callback
argument on the server side is undefined, leading to invalid function
calls (`undefined()`) and an unhandled exception.
The user can also provide lots of other arguments, so the 2nd/3rd ...
argument is of arbitrary type, again leading to invalid function calls
-- e.g. `1()`.
