Silence some gnus compilation warnings

Silence some gnus compilation warnings

* gnus/gnus-ems.el (set-process-plist): Every supported Emacs has this.

* gnus/gnus-group.el (gnus-sequence-of-unread-articles)
(gnus-summary-add-mark, gnus-mark-article-as-read)
(gnus-group-make-articles-read): Declare.

* gnus/gnus-sum.el (gnus-parameter-list-identifier)
(gnus-article-stop-animations, gnus-stop-downloads)
(gnus-article-only-boring-p, article-goto-body)
(gnus-flush-original-article-buffer, article-narrow-to-head)
(gnus-article-hidden-text-p, gnus-delete-wash-type)
(gnus-summary-save-in-pipe, gnus-article-show-summary): Declare.

* gnus/gnus.el: No need to eval-and-compile autoloads.

* gnus/gravatar.el (help-function-arglist): Autoload.

* gnus/nnimap.el (gnus-refer-thread-use-nnir): Declare.

* gnus/nnmail.el (nnmail-fancy-expiry-target): Maybe use mail-dont-reply-to.

* gnus/spam.el: No need to load spam-report when compiling.
No need to eval-and-compile autoloads.
(spam-report-resend-to): Declare.
(spam-report-resend-register-routine): Require 'spam-report.