Add a framework for decrypting enveloped-data
    
    Add logic to the existing parse-everyting PKCS#7 routines to handle
    decrypting enveloped-data.  The NSS version doesn't currently work
    because the library only wants to use certificates in the database to
    locate encryption keys, ignoring session certificates entirely, so it's
    part of this patch only so that it can be picked up again if it's needed
    for something else later.