Handle MS-TNEF PGP/MIME Mails

* src/mapihelp.cpp (get_first_attach_mime_tag): Work with more
then one attachment.
(change_message_class_ipm_note): More ms-tnef handling.

Apparently Office365 converts mails from some to ms-tnef
attachments. We still won't work for S/MIME ms-tnef attachments
as I would need an example, but PGP/MIME should work now
additionally to PGP/Inline.

• GnuPG-Bug-Id: T3802