Mail gateway fixes and improvements. - new mailgw config item unpack_rfc822 that unpacks message attachments of type message/rfc822 and attaches the individual parts instead of attaching the whole message/rfc822 attachment to the roundup issue. - Fix handling of incoming message/rfc822 attachments. These resulted in a weird mail usage error because the email module threw a TypeError which roundup interprets as a Reject exception. Fixes issue2550667. Added regression tests for message/rfc822 attachments with and without configured unpacking (mailgw unpack_rfc822, see Features above) Thanks to Benni Bärmann for reporting.