* nnkiboze.el (nnkiboze-request-article): Use

	* nnkiboze.el (nnkiboze-request-article): Use
	gnus-agent-request-article.

	* nnagent.el (nnagent-retrieve-headers): Don't use nnml
	function. Insert undownloaded NOV.

	* gnus-agent.el (gnus-agent-retrieve-headers): New function.
	(gnus-agent-request-article): New function.

	* gnus.el (gnus-agent-cache): New variable.

	* gnus-int.el (gnus-retrieve-headers): Use
	gnus-agent-retrieve-headers.
	(gnus-request-head): Use gnus-agent-request-article.
	(gnus-request-body): Ditto.

	* gnus-art.el (gnus-request-article-this-buffer): Use
	gnus-agent-request-article.

	* gnus-sum.el (gnus-summary-read-group-1): Don't show the first
	article if it is undownloaded.