bug#37805: gnus-summary-sort-by-fill-in-your-favorite-header

[積丹尼 Dan Jacobson]

Currently there is
C-c C-s C-a     gnus-summary-sort-by-author
C-c C-s C-c     gnus-summary-sort-by-chars
C-c C-s C-d     gnus-summary-sort-by-date
C-c C-s TAB     gnus-summary-sort-by-score
C-c C-s C-l     gnus-summary-sort-by-lines
C-c C-s C-n     gnus-summary-sort-by-number
C-c C-s C-o     gnus-summary-sort-by-original
C-c C-s C-r     gnus-summary-sort-by-random
C-c C-s C-s     gnus-summary-sort-by-subject
C-c C-s C-t     gnus-summary-sort-by-recipient

Well, there also should be
C-c C-s C-f     gnus-summary-sort-by-fill-in-your-favorite-header

Which prompts for and perhaps tab expands a header name.

That way, the user, without needing to write any code, could e.g.,
search for all the messages with a Gnus-Warning header. Thus grouping
together all the messages he wants to delete from bug #32383 .

(And even if Gnus-Warning isn't a real header, it would still be handy
to have this command.)