Merge changes made in Gnus trunk.

mm-decode.el (mm-save-part): Put the entire path in the `M-n' slot.
nnimap.el (nnimap-find-article-by-message-id): Don't EXAMINE a group if it's already selected.
gnus.texi (Customizing the IMAP Connection): Document `nnimap-expunge' and remove `nnimap-expunge-inbox' from example.
gnus.texi (Customizing the IMAP Connection): Remove nnir mention, since that works by default.
gnus-sum.el (gnus-summary-show-article): Take `t' as the arg to mean "raw".
gnus-html.el (gnus-html-browse-url): Implement mailto: URLs.
shr.el (shr-browse-url): Implement mailto: URLs.
nnir.el, gnus-sum.el: Improve thread-referral.
message.el (message-send-mail): Use the value of message-courtesy-message from the message buffer.