spam.el spam- and ham-process-destination improvement

[Ted Zlatanov <tzz@lifelogs.com>]

I added a feature to spam.el, that you can set
spam-process-destination or ham-process-destination on any group.  If
the group's backend allows article moves, the spam or ham will be
moved out.  If the group's backend does not allow article moves, the
spam or ham will be *copied* to the destination.  

This makes it possible to mark spam in nntp groups, for instance, and
copy it to a training group.

I've thought about filtering spam in nntp groups, where new mail is
discovered in each group when you enter it.  I think it can be done
on unseen articles - just apply a spam-mark if spam-split, invoked on
the article, returns the value of spam-split-group.  Opinions?

Thanks
Ted