Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

gnus.texi (Expiring Mail): Document gnus-auto-expirable-marks.
nnir.el (nnir-notmuch-program, nnir-notmuch-additional-switches, nnir-notmuch-remove-prefix, nnir-engines, nnir-run-notmuch): New nnir `notmuch' backend.
mm-decode.el (mm-text-html-renderer): Doc fix.
gnus-msg.el (gnus-bug): Fix the MML tag.
pop3.el (pop3-open-server): -ERR is a valid response to CAPA.
gnus-start.el (gnus-get-unread-articles): Don't connect to the secondary methods if started with `gnus-no-server'.
gnus-group.el (gnus-read-ephemeral-bug-group): Allow fetching several bug reports at once.