std.Io.Threaded: FreeBSD implementation of `netLookup`

[andrewrk]

Extracted from #25592.

The FreeBSD implementation of netLookup should use dnsres_getaddrinfo to obtain this property:

• asynchronously add to results queue

It does not appear to be cancelable.

    if (native_os == .freebsd) {
        // TODO use dnsres_getaddrinfo
    }