Add openssl smime troubleshooting for x509 commits

What does this MR do?

Add some more troubleshooting for x509 / SMIME signing of commits

We assume that the only reason for pkcs7_verify failing is that the root is missing.

unsupported certificate purpose - for one - is not caused by a missing root. And Rails doesn't surface this error.

Dropping down to the command line means folks get a more generic way to test signature verification that's more likely to have good clues from internet searches.

We still refer to some old bugs as limitations. I've added notes about the applicability of these issues.

Related issues

• #440189
• https://gitlab.zendesk.com/agent/tickets/503148 (internal link)
• https://gitlab.zendesk.com/agent/tickets/479456 (internal link)

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