bug#42637: 27.0.91; mm-view-pkcs7 doesn't handle S/MIME signed andd encrypted messages

[Timothy J. Miller]

An S/MIME message that is both enveloped (outer) and signed 
(inner) is not processed correctly. The outer envelop is 
decrypted, but this inserts the signed message into the buffer as 
an opaque blob, like so:

```
Content-Type: application/x-pkcs7-mime; name=smime.p7m; 
smime-type=signed-data
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7m

MIAGCSqGSIb3DQEHAqCAMIACAQExCzAJBgUrDgMCGgUAMIAGCSqGSIb3DQEHAaCAJIAEggEOQ29u
[...]
```


-- T