Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

gnus-sum.el (gnus-update-read-articles): Fix typo.
gnus.el (gnus-valid-select-methods): Mark nnimap as a backend that really have server-side marks.
gnus-sum.el (gnus-propagate-marks): Change default back to nil again, since most backends do not usefully have server-side marks.
 (gnus-update-read-articles): Propagate marks to all backends that really have server-side marks.
message.el (message-cite-reply-position, message-cite-style): New variables.
 (message-yank-original): Use the new citation styles.