On BSD-family systems, header <sys/sockio.h> defines socket ioctl
numbers like SIOCGIFCONF. Only AIX is using those now, but it defines
them in <net/if.h> anyway.
Supposing some PostgreSQL hacker wants to test that AIX-only code path
on a more common development system by pretending not to have
getifaddrs(). It's enough to include <sys/ioctl.h>, at least on macOS,
FreeBSD and Linux, and we're already doing that.
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