[ipv6] Regarding 8220673: Add test library support for determining platform IP support

[Arthur Eubanks]

>
> > Regarding trusting the IPSupport, I have checked to make sure that it's
> reasonable. It correctly throws a jtreg.SkippedException when
> preferIPv4Stack is true and IPv4 is not available, and leaves the other
> cases alone. This was tested with an LD_PRELOAD that returns an error on
> `socket()`, `setsockopt()`, and `getifaddrs()` on IPv4 addresses. Under a
> typical dual stack configuration it doesn't throw anything. This was a good
> idea, thanks Daniel.
>
> Indeed. And thanks for verify its behaviour.
>
I lied, it worked on our JDK which has some IPv6 patches, not the upstream
JDK. I think to properly test this, first I need to upstream a tiny portion
of the patches we have to the JDK. I'll file a bug and send out a patch.
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