Hello!

Since I was able to get the Newsgroups extra header working for
`nnvirtual' groups, I would like to not only limit, but also sort by the
extra headers.

Thanks!

	New Gnus Summary buffer sort options for extra headers

	* lisp/gnus/gnus-sum.el (gnus-article-sort-functions)
	(gnus-thread-sort-functions, gnus-subthread-sort-functions)
	(gnus-summary-mode-map, gnus-summary-make-menu-bar)
	(gnus-article-sort-by-newsgroups)
	(gnus-summary-sort-by-newsgroups, gnus-summary-sort-by-extra):
	Sort by Newsgroups extra header. Prompt for header name for other
	extra headers.

	* doc/misc/gnus.texi (Summary Sorting): Document new sort functions

	* etc/NEWS: New Gnus Summary buffer sort feature