Make the URL API more extensible.

Right now we have no way to add any more parameters beyond a set of
 basic binary flags.
This unifies op_url_stream_create() and
 op_url_stream_create_with_proxy() into a single function that
 takes a variable-length list of arguments, which can be extended
 in the future to include more options of any type.

This is an incompatible API change.