VULN-118896: MacOS service: use --token-file instead of --token

* Refactor directory-related functions in macos_service.go to pull out common logic
* Use --token-file instead of --token when installing cloudflared with a token on MacOS such that the secret token will not show up in the output of "ps aux" (or similar)
* Improve the "service install" help text on MacOS to distinguish between the "service install " and "service install \<token\>" cases