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	connection is open before saving it.
	(url-http-handle-authentication): Use url-retrieve-internal
	instead of url-retrieve.
	(url-http-parse-headers): Adapt to new callback interface.
	(url-http): Handle non-blocking connections.
	(url-http-async-sentinel): Create.

	* url.el (url-retrieve): Update docstring for new callback interface.
	Remove all code.
	(url-retrieve-internal): Move code from url-retrieve here.

	* url-gw.el (url-open-stream): Use a non-blocking socket for
	`native' gateway method, if available.