Create a global daemon instead of a user agent on macOS

[JacobHenner]

When VSTS is installed as a user agent (through svc.sh), the service is only available when the VSTS user has logged into the GUI. Running the service as a global daemon (as the VSTS user) would prevent the need for the user to login to the GUI.

(However, this is not currently a MSFT supported arrangement, and several types of build activities requiring a GUI may be limited)

[JacobHenner]

If this is implemented, it'll be an parameterized option, and the standard VSTS user agent creation process will still be available.

[JacobHenner]

microsoft/azure-pipelines-agent#1843

[JacobHenner]

This will not be handled upstream. Will try to add functionality to the module.