Merge changes made in Emacs trunk.

auth.texi: Sync @dircategory with Emacs/info/dir.
emacs-mime.texi: Sync @dircategory with Emacs/info/dir.
gnus.texi: Sync @dircategory with Emacs/info/dir.
message.texi: Sync @dircategory with Emacs/info/dir.
pgg.texi: Sync @dircategory with Emacs/info/dir.
sasl.texi: Sync @dircategory with Emacs/info/dir.
sieve.texi: Sync @dircategory with Emacs/info/dir.
gnus.el (gnus-meta): Doc fix.
nnfolder.el (nnfolder-save-buffer): Don't let-bind copyright-update, in case it's not yet loaded.