Render all parts of multipart/encrypted when decrypting.

The primary goal here is to keep the decrypted output as similarly
structured as undecrypted output as possible.  Now, when decrypting
parts, only the original encrypted part is replaced by the it's
decrypted content.  If this part isn't itself a multipart, then all
part numbering should remain consistent during decryption.

The only draw back here is that the useless application/pgp-encrypted
sub-part of the multipart/encrypted part is also emitted.  But this
part can be easily ignored by clients.