Activated two-part nntp async fetch again

[Lars Ingebrigtsen <larsi@gnus.org>]

I've now activated the two-part NNTP async fetch again, so your Gnus
will hang again, if you were affected by the hangs earlier.

However, I think I may have made some headway in trying to figure out
why it hangs, although I've been unable to reproduce the problem myself.

I think the issues is that `nntp-retrieve-group-data-early' is called
twice on the same server, for some reason.  And that makes all the
output go into the same buffer, and things get really messed up.

If you are affected by this problem, could you `M-x debug-on-entry RET
nntp-retrieve-group-data-early RET' and then note what the value of the
`server' variable is in each instance?  If you get the same value twice
in the same `g' session, then we've found the culprit, and I'd like to
have the backtraces from two of those calls so that I can see what's
screwing things up.

-- 
(domestic pets only, the antidote for overdose, milk.)
  larsi@gnus.org * Lars Magne Ingebrigtsen