* gnus-agent.el (gnus-agent-cat-groups): rewrote avoiding defsetf

	* gnus-agent.el (gnus-agent-cat-groups): rewrote avoiding defsetf
	to avoid run-time CL dependencies.
	(gnus-agent-unfetch-articles): New function.
	(gnus-agent-fetch-headers): Use gnus-agent-braid-nov to validate
	article numbers even when local .overview file is missing.
	(gnus-agent-read-article-number): New function. Only accepts
	27-bit article numbers.
	(gnus-agent-copy-nov-line, gnus-agent-uncached-articles): Use
	gnus-agent-read-article-number.
	(gnus-agent-braid-nov): Rewrote to validate article numbers coming
	from backend while recognizing that article numbers in .overview
	must be valid.
	(gnus-agent-update-files-total-fetched-for): Use
	directory-files-and-attributes to improve performance.