Merge changes made in Gnus master

Merge changes made in Gnus master

nnimap.el (nnimap-authenticator): Expand to allow specifying the
  login methods.
  (nnimap-login): Respect the `nnimap-authenticator' variable.
gnus-sum.el (gnus-summary-push-marks-to-backend): Push the complete
  mark state when moving articles.  Otherwise unticked articles will get
  their ticks back after moving.
gnus.texi (Customizing the IMAP Connection): Mention the other
  authenticators.