* 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.