util/repair: identify and repair "Mixed Up" mangled messages

Implement a functional identification and repair process for "Mixed
Up" MIME messages as described in
https://tools.ietf.org/html/draft-dkg-openpgp-pgpmime-message-mangling-00#section-4.1

The detection test is not entirely complete, in that it does not
verify the contents of the latter two message subparts, but this is
probably safe to skip, because those two parts are unlikely to be
readable anyway, and the only part we are effectively omitting (the
first subpart) is guaranteed to be empty anyway, so its removal can be
reversed if you want to do so.  I've left FIXMEs in the code so that
anyone excited about adding these additional checks can see where to
put them in.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>