Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

nnimap.el (nnimap-wait-for-response): Always look (at least) at the previous line.
nnimap.el (nnimap-quirk): New function.
 (nnimap-retrieve-group-data-early): Use it.
 (nnimap-quirks): New alist.
gnus.texi (Foreign Groups): Added clarification of foreign groups.
gnus-sum.el (gnus-summary-push-marks-to-backend): Fix the logic for copying read-ness to the backends.
gnus-group.el (gnus-group-kill-group): Notify the backend that the group has been killed.
 (gnus-group-yank-group): Ditto.
gnus-start.el (gnus-subscribe-newsgroup): Notify the backend.
nnir.el: Improve customizations.
gnus.texi (Archived Messages): Removed outdated comment and text.
nnfolder.el (nnfolder-save-all-buffers): Refactor out into its own function.
 (nnfolder-request-expire-articles): Save all the buffers after doing expiry.
nnmail.el (nnmail-expiry-target-group): Revert the "all articles are the last article", since that led to serious performance regressions when expiring nnml groups.
gnus-html.el (gnus-html-schedule-image-fetching): Make sure the HTML fetching stops when Gnus exits.
gnus-srvr.el: Avoid passing nil regexp argument to delete-matching-lines.
auth-source.el (auth-source-gpg-encrypt-to): New variable to set the list of recipient keys, or use symmetric encryption if not a list.
 (auth-source-create): Use it to make `epa-file-encrypt-to' local for an EPA override, replacing the call to `netrc-store-data'.