create users and groups on Darwin when building

[awanwar]

Was unable to create users and groups on macOS because "groupadd" and "useradd" only work on Linux. I've added a patch that detects if you're building on Darwin, and if you are, create the appropriate user and group using Darwin-specific commands. I set the user ID and the group ID both to 601. Feel free to change them, but be sure that no two users (or groups) will have the same ID by making it uncommon or adding some kind of script to detect unused IDs.