Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

gnus-util.el: change default value of gnus-user-date-format-alist.
gnus-art.el (gnus-article-jump-to-part): Remove useless sit-for.
gnus-art.el: remove old FIXME.
gnus.el (gnus-summary-line-format): Add missing semi-colon for user-date in docstring.
message.el (message-setup-1): Always generate References first.
 (message-mail): Return the return value of message-setup, not always t.
gnus-start.el (gnus-read-active-for-groups): This function is never called with a nil `infos', so clean that up.
 (gnus-get-unread-articles): Request active files from primary/secondary methods that have no groups (yet).
 (message-setup-1): Insert mail-header-separator with read-only and intangible properties set.
gnus-draft.el: Remove progn around gnus-draft-setup.
gnus-start.el (gnus-get-unread-articles): Fix the call to methods that have no groups.
nnimap.el (nnimap-request-accept-article): Give an error message if the APPEND wasn't successful.