(gnus-registry-marks, gnus-registry-default-mark):

(gnus-registry-marks, gnus-registry-default-mark):
Add new variables for article mark management.
(gnus-registry-extra-entries-precious, gnus-registry-trim): Define a
list of extra data entries which, when present, will indicate that the
article ID should not be trimmed from the registry.
(gnus-registry-mark-article, gnus-registry-article-marks): Remove these functions.
(gnus-registry-read-mark): New function to read a mark name from the user.
(gnus-registry-set-article-mark, gnus-registry-remove-article-mark)
(gnus-registry-set-article-mark-internal): New functions to add and
remove marks.
(gnus-registry-get-article-marks): New function to show the marks for
an article, or retrieve them for further use.