WIP: fastopen: only use connectx if available

Add a description of the new feature/bug fix. Reference any relevant bugs.

Checklist

• Code modified for feature
• Test suite updated with functionality tests => This need a pipeline with this version of OS X, see #662 (closed)
• Test suite updated with negative tests => This should not be changed
• Documentation updated / NEWS entry present (for non-trivial changes) => I don't think I should change anything, trivial change.

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code