Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

nnmail.el (nnmail-get-new-mail-1): If one mail source bugs out, continue on and do the clean-up phase (bug#9188).
gnus-sum.el (gnus-summary-expire-articles): When expiring articles, just ignore groups that can't be opened instead of erroring out (bug#9225).
gnus-art.el (gnus-article-update-date-headers): Flip the default to nil since some many people are fuddy-duddies.
gnus-html.el (gnus-html-image-fetched): Don't cache zero-length images.