next ifreq is sizeof(struct ifreq) after the current one unless the
sockaddr is bigger than the union in ifreq that holds it.
In the original 4.4BSD code, this interpretation results in the same
behavior as the "is the sockaddr bigger than struct sockaddr", because
sizeof(struct sockaddr) and sizeof(ifc->ifr_ifru) are the same.
Add comments pointing out problems in the 'need bigger buffer' code,
and copying excessive amounts of data.
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