merge emacs Gnus changes -> Gnus 5.10.6 branch

[Miles Bader]

I've merged Stefan's post-5.10-merge Gnus changes from the Emacs trunk
into the Gnus v5-10 branch (they all look pretty reasonable to my quick
eyeballing).

I'll merge any future Emacs Gnus changes similarly into the Gnus v5-10
branch unless they seem questionable (by the eyeball test), in which
case I'll try to ask on the ding list.

I've also done a test-merge of these and other v5-10 changes into a
local copy of the Gnus CVS trunk.  There are a few conflicts I'm going
to look at more closely (e.g., lots of duplicate ChangeLog entries
because of previous duplicate commits to the v5-10 branch and the
branch), but I think most of these are straight-forward and
uncontroversial; I'll post a patch with anything I'm not sure about.

My intent with this is to essentially keep both the v5-10 branch and
Gnus CVS trunk as close to a `plop into Emacs' state as possible, with
deviations explicitly kept track of.

Thanks,

-Miles
-- 
`Suppose Korea goes to the World Cup final against Japan and wins,' Moon said.
`All the past could be forgiven.'   [NYT]