Merge changes made in Gnus master

Merge changes made in Gnus master

gnus-srvr.el (gnus-browse-delete-group): Fix syntax error.
message.el (message-ignored-news-headers): Always remove
  X-Message-SMTP-Method to avoid information leakage if the user
  mistakenly inserts the header into news messages.
gnus-srvr.el (gnus-browse-delete-group): New command and keystroke.
gnus-sum.el (gnus-summary-hide-thread): If point were further to the
  right than four characters, this command would move point to
  `point-max'.  Don't do that.
gnus-group.el (gnus-group-read-ephemeral-group): Set the active data
  to nil to allow re-selecting groups that gain articles.
  (gnus-bug-group-download-format-alist): Update the URL.
gnus.texi (Browse Foreign Server): Document `gnus-browse-delete-group'.