Allow use as SYSTEM

[cybermoloch]

This is a further iteration on the last PR in which I've added a way to run the script from the SYSTEM context and will rerun the command as the logged in user. This makes it easier to protect the client secret as they can be pulled from a protected registry key or similar with reduced read permissions. (I'll be making a third version for this which will pull from the registry but I don't think it would make sense as a PR since it will be specific to the software [Datto RMM and PolicyPak] I am deploying it with.)

I've submitted this as a separate PR as I am not sure this is a desired function in your use case.