Make sure that all gnus buffers are killed on exit

Make sure that all gnus buffers are killed on exit

* lisp/gnus/gnus-agent.el (gnus-agent-synchronize-flags-server):
Use `gnus-get-buffer-create' instead of `get-buffer-create'
* lisp/gnus/gnus-bookmark.el (gnus-bookmark-write-file): Likewise.
(gnus-bookmark-bmenu-list): Likewise.
(gnus-bookmark-show-details): Likewise.
* lisp/gnus/gnus-draft.el (gnus-draft-setup): Likewise.
* lisp/gnus/gnus-icalendar.el (gnus-icalendar-reply): Likewise.
* lisp/gnus/gnus-int.el (gnus-backend-trace): Likewise.
* lisp/gnus/gnus-srvr.el (gnus-enter-server-buffer): Likewise.
* lisp/gnus/gnus-sum.el (gnus-summary-pipe-output): Likewise.
* lisp/gnus/gnus-util.el (gnus-output-to-rmail): Likewise.
(gnus-output-to-mail): Likewise.
(gnus-multiple-choice): Likewise.
* lisp/gnus/mail-source.el (mail-source-call-script): Likewise.
* lisp/gnus/message.el (message-cancel-news): Likewise.
(message-resend): Likewise.
* lisp/gnus/mm-archive.el (mm-dissect-archive): Likewise.
* lisp/gnus/mml1991.el (mml1991-mailcrypt-sign): Likewise.
(mml1991-mailcrypt-encrypt): Likewise.
* lisp/gnus/mml2015.el (mml2015-mailcrypt-verify): Likewise.
(mml2015-mailcrypt-clear-verify): Likewise.
* lisp/gnus/nnbabyl.el (nnbabyl-request-move-article): Likewise.
* lisp/gnus/nndiary.el (nndiary-request-move-article): Likewise.
(nndiary-find-group-number): Likewise.
(nndiary-open-nov): Likewise.
(nndiary-generate-nov-file): Likewise.
* lisp/gnus/nndoc.el (nndoc-possibly-change-buffer): Likewise.
* lisp/gnus/nndraft.el (nndraft-request-move-article): Likewise.
(nndraft-auto-save-file-name): Likewise.
* lisp/gnus/nneething.el (nneething-get-head): Likewise.
* lisp/gnus/nnfolder.el (nnfolder-request-move-article): Likewise.
(nnfolder-open-nov): Likewise.
* lisp/gnus/nnheader.el (nnheader-init-server-buffer): Likewise.
(nnheader-set-temp-buffer): Likewise.
* lisp/gnus/nnimap.el (nnimap-log-buffer): Likewise.
* lisp/gnus/nnir.el (nnir-run-swish++): Likewise.
(nnir-run-swish-e): Likewise.
(nnir-run-hyrex): Likewise.
(nnir-run-namazu): Likewise.
(nnir-run-notmuch): Likewise.
(nnir-run-find-grep): Likewise.
* lisp/gnus/nnmail.el (nnmail-split-incoming): Likewise.
(nnmail-cache-open): Likewise.
(nnmail-log-split): Likewise.
* lisp/gnus/nnmaildir.el (nnmaildir--with-work-buffer): Likewise.
(nnmaildir--with-nov-buffer): Likewise.
(nnmaildir--with-move-buffer): Likewise.
* lisp/gnus/nnmairix.el (nnmairix-call-mairix-binary): Likewise.
(nnmairix-call-mairix-binary-raw): Likewise.
(nnmairix-replace-group-and-numbers): Likewise.
* lisp/gnus/nnmbox.el (nnmbox-request-move-article): Likewise.
* lisp/gnus/nnmh.el (nnmh-request-move-article): Likewise.
* lisp/gnus/nnml.el (nnml-request-move-article): Likewise.
(nnml-find-group-number): Likewise.
(nnml-get-nov-buffer): Likewise.
(nnml-generate-nov-file): Likewise.
* lisp/gnus/nnrss.el (nnrss-opml-export): Likewise.
* lisp/gnus/nntp.el (nntp-record-command): Likewise.
* lisp/gnus/nnvirtual.el (nnvirtual-retrieve-headers): Likewise.
* lisp/gnus/smime.el (smime-new-details-buffer): Likewise.
(smime-certificate-info): Likewise.
(smime): Likewise.
* lisp/gnus/spam-stat.el (spam-stat-store-current-buffer): Likewise.