When running as root, use build users by default

This removes the need to have a nix.conf, and prevents people from
accidentally running Nix builds as root.
Set build-max-jobs to the number of available cores by default

More zero configuration.
Install an Upstart service
Set up directories and permissions for multi-user install automatically

This automatically creates /nix/var/nix/profiles/per-user and sets the
permissions/ownership on /nix/store to 1775 and root:nixbld.