normalizing part numbering across PGP/MIME processing

[Jameson Graef Rollins]

Ok.  So I very much hope this patch series satisfies those who were
bothered by the part renumbering that was happening when PGP/MIME
parts were processed.  For signed messages we no longer modify the
parts at all, so numbering always remains constant, and for encrypted
messages the numbering will only change if the encrypted message is
itself multipart.  We'll have to live with the encrypted multipart
renumbering until we have a more sophisticated part numbering scheme.
However, that's a separate issue that can hopefully be dealt with
after the full crypto functionality is pulled.

jamie.