Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

gnus.el: Remove `gnus-nntp-service' variable.
gnus.el: Make gnus-nntp-server and gnus-secondary-servers obsolete.
gnus-sum.el (gnus-summary-delete-marked-as-read, gnus-summary-delete-marked-with): Remove obsolete defalias.
gnus.el (gnus-use-long-file-name): Fix docstring.
nnimap.el (nnimap-open-connection): Use AUTHENTICATE PLAIN on servers that say they support that.
gnus-msg.el (gnus-inews-do-gcc): Don't have the backends do the slow *-request-group, which seems unnecessary.
gnus-group.el (gnus-group-get-new-news-this-group): Don't have point move to the previous line on `M-g'.
nnimap.el (nnimap-split-incoming-mail): Note that the INBOX has been selected.
nnimap.el: Allow the user to say whether to split old messages or not in nnimap.
shr.el (shr-tag-table-1): Only insert the images after the top-level table.
shr.el (shr-tag-span): Drop colorisation of regions since we don't control the background color.
shr.el (shr-tag-img): Ignore very small web bug type images.
shr.el (shr-put-image): Add help-echo alt texts to the images.
shr.el (shr-tag-video): Show the video poster image.