New Gnus Summary buffer sort options for extra headers

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 (bug#49081).