Safe and useful handling of "Mixed Up" mangled messages

[Daniel Kahn Gillmor <dkg@fifthhorseman.net>]

I've documented an unfortunate MTA habit over in
https://tools.ietf.org/html/draft-dkg-openpgp-pgpmime-message-mangling-00#section-4.1
which i've named "Mixed Up" mangling.  In particular, popular versions
of Microsoft Exchange take a multipart/encrypted e-mail and transform
it unaccountably to multipart/mixed.

While the right thing to do long term is to get a fix for those MTAs
(which i'm also working on, elsewhere), even if we succeed in doing
that, messages that have already been mangled by them will live on in
our mailboxes forever.

What follows is a series of patches that lets a notmuch user deal with
any message mangled in this particular form in a sensible way.  In
particular, they should be able to decrypt the message correctly
despite its nonstandard structure.  This represents a deviation from
standard MIME handling procedures, but i believe it is a safe
deviation, and a useful one.

The test case included in this series should be sufficient to show the
problem specifically, but if anyone wants to receive encrypted e-mail
from me that demonstrates the problem to their personal key, i can
provide.  Please let me know by replying off-list and making sure i
know your OpenPGP key fingerprint.

If anyone has examples of other common, detectable, and repairable
message manglings, please let me know.  I'd be happy to add them to my
documentation of this sort of thing at least, and if we can fix them
up for notmuch users, even better.

All the best,

    --dkg