Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

shr.el (shr-render-td): Use a cache for the table rendering function to avoid getting an exponential rendering behaviour in nested tables.
shr.el (shr-insert): Rework the line-breaking algorithm.
shr.el (shr-insert): Don't leave trailing spaces.
shr.el (shr-insert-table): Also insert empty TDs.
shr.el (shr-tag-blockquote): Ensure paragraphs after </ul>.
gnus-start.el (gnus-get-unread-articles): Require gnus-agent before bidning gnus-agent variables.
mm-decode.el (mm-save-part): If given a non-directory result, expand the file name before using to avoid setting mm-default-directory to nil.
gnus.el (gnus-carpal): The carpal mode has been removed, but define the variable for backwards compatability.
nnimap.el (nnimap-update-info): Remove double setting of high.
nnimap.el (nnimap-update-info): Don't ignore groups that have no UIDNEXT.
shr.el (require): Require cl when compiling.
shr.el (shr-tag-hr): New function.