Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

gnus.texi (Store custom flags and keywords): Refer to `gnus-registry-article-marks-to-{chars,names}' instead of `gnus-registry-user-format-function-{M,M2}'.
shr.el (shr-rescale-image): Add an :ascent of 100 to images so that the underline comes at the bottom.
gnus-registry.el (gnus-registry-article-marks-to-chars): Rename from `gnus-registry-user-format-function-M' and declare the latter obsolete.
 (gnus-registry-article-marks-to-names): Rename from `gnus-registry-user-format-function-M2'.
shr.el (shr-browse-image): Copy the URL if called interactively.