Fix multipart/signed detection if ms-tnef wrapped

* src/mapihelp.cpp (change_message_class_ipm_note_smime),
(change_message_class_ipm_note_smime_multipartsigned): Handle tnef.

This fixes mutlipart/signed detection both when s/mime is enabled
and when it's disabled. For these kind of ms-tnef wrapped mails
there is actually a virtual attachment with a filename smime.p7m
that contains our mimestructure. Happlily all our other code
can handle this already.

• GnuPG-Bug-Id: T3802