MH should deal with multipart/mime where some parts are signed.

[massar]

if multipart/signed, demote that to one leg of a multipart/mixed, and make
the disclaimer text another leg of that multipart/mixed. this should preserve
S/MIME for people that use it. [maybe always-demote, and make
sure the original string remains intact, by adding our disclaimer to a new
top-level multipart/mixed?]