prune the overly cavalier "kill all gnus buffers exit"

prune the overly cavalier "kill all gnus buffers exit"

* lisp/gnus/mail-source.el (mail-source-call-script): Require gnus for
 `gnus-get-buffer-create', following the pattern in the file.
* lisp/gnus/message.el: Autoload `gnus-get-buffer-create'.
* lisp/gnus/mm-archive.el: Likewise.
* lisp/gnus/mml2015.el: Likewise (the file autoloads other gnus functions).
* lisp/gnus/nnheader.el: Likewise.
* lisp/gnus/mml1991.el (mml1991-mailcrypt-sign, mml1991-mailcrypt-encrypt):
Revert the patch, use `get-buffer-create' instead of `gnus-get-buffer-create'.
* lisp/gnus/smime.el (smime-new-details-buffer, smime):
 smime-certificate-info): Likewise.
* lisp/gnus/spam-stat.el (spam-stat-store-current-buffer): Likewise.