Clean up MS-TNEF handling.

Clean up MS-TNEF handling.

Don't show winmail.dat entry - instead, make any embedded attachments
look like attachments to the main e-mail message.  The one negative - it
is not possible to strip the attachments from the message since
winmail.dat no longer appears in attachment lists.  But IMHO this is a
fair tradeoff to reduce complexity/confustion when showing the message
parts (and ms-tnef is not really a valid/viable standard anyway).