looking for references headers for the ^ command

[Greg Stark <gsstark@MIT.EDU>]

If the NOV informatio contains a referencesentry but the actual article is
missing it Gnus seems to do somewhat unpredictable things, sometimes ^ works
sometimes it doesn't. This situation arises because my backend interfaces to a
system that implements threading but not via references headers. So i generate
synthetic message-id and references headers for the nov data but i am loath
the actually fiddle with the contents of the articles, especially since some
of them contain real message-id's and references headers and i wouldn't want
my synthetic headers to ever leak out.

I think the right thing is for gnus to try any references headers provided by
either source, the NOV data and the actual headers. I'm not sure which it
should prefer if both return message-id's for available articles, perhaps it
should be a variable the backend could tweak. 

I'll look into doing this myself since it's likely it only affects me :)
but i just though i would mention it in case someone else is concerned.

greg