Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

pop3.el (pop3-open-server): Upgrade opportunistically to STARTTLS.
 (open-tls-stream): Remove superfluous tls/starttls autoloads.
gnus-sum.el (gnus-summary-next-article): Don't bug out if the summary buffer has moved to a different frame.
nnimap.el (nnimap-request-article): Use nntp-insert-buffer-substring to get the conversion from unibyte to multibyte buffers to work on Emacs 22.
nntp.el (nntp-request-article): Slight clean-up.