Merge changes made in Gnus trunk:

gnus-html.el (gnus-html-schedule-image-fetching): Decode entities before feeding URLs to curl.
gnus-async.el (gnus-async-article-callback): Call `gnus-html-prefetch-images' unconditionally.

gnus-html.el: Allow showing the ALT text of images and to browse the images themselves.

gnus-html (gnus-html-wash-tags): Search for images first, so that <a><img> works better; (gnus-html-displayed-image-map): Bind RET and TAB on images for better UX.