This patch lets functions registered in the
`summary-article-expire-hook' know which group the expired articles are
going to. In particular this is useful for the registry: if people are
using expiration to move articles to a different group, rather than
deleting them, the registry should know where they've gone.

If nnmail-expiry-target is a function, this will still work, but not
quite as the docstring says: the hook function is given the from-group
as an argument, but is *not* called in a buffer narrowed to the message,
as it says. That would be hard to do, though, and this seems better than
nothing.

Eric