Get the hardware address as a 48-bit positive integer.
The first time this runs, it may launch a separate program, which could
be quite slow. If all attempts to obtain the hardware address fail, we
choose a random 48-bit number with its eighth bit set to 1 as recommended
in RFC 4122.
3.7: Universally administered MAC addresses are preferred over locally administered MAC addresses, since the former are guaranteed to be globally unique, while the latter are not.