It's likely most GNU/Linux systems have /etc/machine-id these
days, so anything missing it is likely a *BSD, most of which
support and favor "sysctl -n kern.hostid". We'll also support
"ghostid" since GNU utils are commonly prefixed with 'g' on
non-GNU platforms.
In any case, we'll suppress stderr from missing commands and
fall back to hard coding an $OSNAME-based identifier as a last
resort and hope the hostname is unique.