Cross-platform machine ID

[bchopson]

The current code to fetch the machine ID looks in /var/lib/dbus/machine-id.
On a Mac, this command gets the machine ID:
ioreg -d2 -c IOPlatformExpertDevice | awk -F '"' '/IOPlatformUUID/ {print $4}'

[rsyring]

Feel free to open a PR for this. I don't mind tackling it eventually but want to focus on getting the server migrations completed in the near term.