The mailing list monthly archives were all downloading with the same filename, just "psql-hackers.mbox", which turned into "psql-hackers(1).mbox", "psql-hackers(2).mbox", etc.
This patch sets the `Content-Disposition` header to a filename that includes the list and the year/month.
I like this idea and the patch looks good. However, is there a reason not to also do this for the other code path calling _build_mbox()? If nothing else, for consistency?
