Fix mtls option location in s_client_options() and code cleanup

[akabe1]

Describe your changes

This PR changes the position of the $MTLS option into s_client_options(), and cleans some duplicate calls to the openssl option -cert $MTLS in run_http_header(), sub_session_resumption(), and sub_breach_helper() .

What is your change 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]

Looks good to me, thanks! (@dcooper16 : pls raise any concerns you might have)