Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

nnir.el (nnir-categorize): Replace mapcar with mapc.
shr.el (shr-urlify): Display the title in <a> tags.
shr.el (shr-urlify): Show the URL before the title to avoid misleading URLs.
gnus-sum.el (gnus-summary-show-article): Reverse the meanings of `C-u
C-u g' and `C-u g' so that `C-u g' does what it traditionally did.
gnus.texi (Paging the Article): Note the reverse meanings of `C-u C-u g'.
gnus-html.el (gnus-html-put-image): Use widget instead of local maps
so that TAB works.
nnir.el (nnir-run-gmane): Use more careful test for gmane nntp server.
nnimap.el (nnimap-process-expiry-targets): Avoid downloading articles
unless necessary.
gnus-util.el (gnus-output-to-mail): Require nnmail before using nnmail
variables.
shr.el (shr-stylesheet): New dynamic variable for cascading the styles.
 (shr-colorize-region): New function.
 (shr-insert-background-overlay): Remove.
 (shr-render-td): Background setting should be taken care of on a higher level.
 (shr-tag-body): Use post-hoc colorizations.
 (shr-descend): Only render color/background when they change.
 (shr-tag-body): Set up a style sheet based on bgcolor/fgcolor.
 (shr-put-color-1): Don't overwrite old colors.
 (shr-colorize-region): When the background color isn't explicit, use a fixed background.
gnus.el (gnus-valid-select-methods): Allow nnimap to respool.
nntp.el (nntp-snarf-error-message): nnheader-report takes a format string as the parameter.
gnus-sum.el (gnus-summary-respool-article): The completion function expects a list instead of an alist.