Fix check for OpenSSL supported curves

[dcooper16]

Describe your changes

OpenSSL 3.X outputs a different error message than previous versions when $OPENSSL s_client -curves X ... is called with an unsupported curve. This was resulting in the check within find_openssl_binary() adding every curve to $OPENSSL_SUPPORTED_CURVES, even ones that were not supported. This PR changes to check in order to detect the new error message.

I have tested the fix against multiple versions of OpenSSL/LibreSSL and verified that $OPENSSL_SUPPORTED_CURVES is being set correctly.

What is your pull request about?

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Typo fix
• Documentation update
• Update of other files

If it's a code change please check the boxes which are applicable

• For the main program: My edits contain no tabs and the indentation is five spaces
• I've read CONTRIBUTING.md and Coding_Convention.md
• I have tested this fix against >=2 hosts and I couldn't spot a problem
• I have tested this new feature against >=2 hosts which show this feature and >=2 host which does not (in order to avoid side effects) . I couldn't spot a problem
• For the new feature I have made corresponding changes to the documentation and / or to help()
• If it's a bigger change: I added myself to CREDITS.md (alphabetical order) and the change to CHANGELOG.md

[drwetter]

Excellent & thx!