Make `J u' fuctional in summary buffer

[Harry Putnam <reader@newsguy.com>]

Gnus has a handy dual buffer usage for the Agent command `J j'
(toggle plugged or unplugged)  It can be used in Group as well as
Summary buffer.

Is it also possible to run `J u'  in summary buffer?  It seems not.

However it would make good sense to have that Command available in
Summary buffer so one could Mark a message for download, go plugged and
download it, all from Summary buffer.

Currently it seems necessary to exit the summary buffer to run `J u'.

I fairly often find myself wanting a post that didn't answer my agent
category, but I have adjusted that predicate/score  about as well as
it can be.

It would be nice if one could immediately download a missing message
from the summary buffer to disk, buy having access to `J u' in Summary
buffer.