nnml.el, nnfolder.el, nntp.el (*-request-set-mark): Extend syntax with 'set.

nnml.el, nnfolder.el, nntp.el (*-request-set-mark): Extend syntax with 'set.
nnheader.el, nntp.el, nnfolder.el, nnml.el (*-request-set-mark): Refactor out nnheader-update-marks-actions and use it throughout.
nnmaildir.el (nnmaildir-request-set-mark): Be explicit about 'set.
gnus-sum.el (gnus-summary-push-marks-to-backend): Use 'set instead of 'add and 'delete to set backend marks.