portability: patch mac os dns issues

[oliver-sanders]

closes #2689
downstream siblings:

• portability: patch mac os dns issues cylc-doc#262
• support all unix conda-forge/cylc-flow-feedstock#25
• support all unix conda-forge/cylc-uiserver-feedstock#17
• support all unix conda-forge/metomi-rose-feedstock#3
• support all unix conda-forge/cylc-rose-feedstock#1

Ok, I've finally given in and patched the DNS logic for Mac OS.

I started a branch overhauling cylc.flow.hostuserutil by making the contained logic completely configurable, however, it turns out to be messier than expected and we don't presently have the time to go down that road - https://github.com/oliver-sanders/cylc-flow/tree/dns

So, simple solution, patch the Cylc logic to handle this strange python-socket binding behaviour and move on.

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.py and
  conda-environment.yml.
• Already covered by existing tests.
• Appropriate change log entry included.
• (master branch) I have opened a documentation PR at portability: patch mac os dns issues cylc-doc#262

[kinow]

The new code is executed only on macos and the logic makes sense. Great comments, CI green, LGTM 🎉 thanks!

[wxtim]

• Looks sane
• GHA tests passing
• Played with it on a Mac VM on my PC.