Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

gnus-group.el (gnus-group-read-ephemeral-group): Make sure we don't enter invalid buffer configurations into the quit form (bug#9107).
 (gnus-group-tool-bar-gnome): Replace connect/disconnect with unplugged/plugged.
gnus-sum.el (gnus-summary-refer-thread): When inserting new headers, keep track of which ones are unread (bug#9061).
gnus.el (gnus-refer-article-method): Allow entering any sexp (bug#9055).
gnus-art.el (gnus-article-show-images): Allow working if using w3m (bug#9041).
gnus-html.el (mm-util): Require (bug#9073).
gnus-sum.el (gnus-delete-duplicate-headers): New function.
 (gnus-summary-refer-thread): Use it to remove duplicates in the un-threaded view (bug#9053).
 (gnus-summary-insert-subject): Document USE-OLD-HEADER (bug#9070).
nnir.el (nnir-read-server-parm): Use default value from global variable.  Without this the default search engine parameters aren't used at all.
message.el (message-unique-id): Don't use the undocumented return value from (random t) (bug#9118).