bug#63157: [EXT] bug#63157: Acknowledgement (29.0.60; mml-sec with EPA S/MIME signencrypt messgages are not parsed by other implementations)

[Timothy J. Miller]

OK, my bad re: gpgsm; I didn't grok the limits of that tool.  So strike
the "anything else" and replace it with "Outlook".  From my mucking
about:

- Clients:
  - gnus-article-mode and descendents
  - TBird
  - Outlook
  - OpenSSL smime
  - gpgsm [with p7s/p7m mime-part properly extracted]

- Signed only messages validate with all clients

- Encrypted only messages decrypt with all clients

- Sign+encrypt messages fail with Outlook.


Outlook is completely unhelpful here; with debug enabled via `defaults
write com.microsoft.Outlook LogForTroubleshooting -bool TRUE` on macOS,
this is about all I can find that's relevant:

```
05/01/2023 07:52:14.812 OUTLOOK (0x118c) 0xc6a8000 Microsoft Outlook 
outlook.olxhx 9tals Unexpected Actor API loadSMIMEContent succeeded but failed 
to unpack message
05/01/2023 07:52:14.812 OUTLOOK (0x118c) 0xc6a8000 Microsoft Outlook 
outlook.mail 4y8j3 Medium PreviewPane loadSMIMEContent - STOP 
{"DecodingIsSuccessful": false}
```

Outlookin
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