Add check for root and use Apple best practice

[pboushy]

The current version of the uninstall script:

1. Doesn't contain an interpreter directive
2. Requires root but doesn't check that it's running as root.
3. Uses a method that is not recommended by Apple for detecting current desktop user.

This PR:

1. sets the directive to be #!/bin/sh to ensure the most compatibility across macOS versions/environments.
2. Checks that the script is being run as root or via sudo
3. Switches to using the POSIX shell compliant version of scutil parsing as documented by scriptingosx in https://scriptingosx.com/2020/02/getting-the-current-user-in-macos-update/

[almenscorner]

Looks good, I updated the console_user_uid as it otherwise was not able to unload the LaunchAgent