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This is a backport of the minimal changes from master (flan) to fix IPv6
multicasting. Specifically, it fixes NetworkInterface to report IPv6 addresses,
it fixes GenericIPMreq so we pass the interface indexes down to native code,
it replaces our old copy of harmony's MulticastSocketTest with the current
upstream version (to avoid bogus failures), and it brings back one small
"unrelated" fix to OSNetworkSystem.cpp that's necessary to prevent failures
in later parts of tests we used to fail too early to notice secondary
problems.

This passes all the (fixed) MulticastSocketTest tests, causes no regressions
in the whole net.AllTests suite, and fixes the user-submitted application
that started the investigation.

Bug: 1750581