Merge changes made in Gnus trunk.

Merge changes made in Gnus trunk.

nnir.el (nnir-get-active): Ignore nnir-ignored-newsgroups if null.
gnus-sum.el (gnus-summary-refer-thread): Simplify code. Restore gnus-use-agent.
 (gnus-get-newsgroup-headers): Avoid unwanted spaces at eol.
gravatar.el (gravatar-retrieve-synchronously): New function.
 (gravatar-get-data): Make more robust.
gnus-util.el (gnus-rescale-image): Allow to resize images even if they are from file. Can also scale up.
nnimap.el (nnimap-retrieve-headers): Remove CRLF from the headers.
nnir.el (nnir-mode): Make sure 'gnus-registry-install is bound.
gnus-sum.el (gnus-get-newsgroup-headers): Revert the last change here, since it's up to the backends to do CRLF removal if their protocol has it.
gnus-group.el (gnus-group-delete-articles): New command.