8220493: Prepare Socket/ServerSocket for alternative platform SocketImpl

[Chris Hegarty]

> ...
> 
> The webrev with the corresponding changes is here:
>    http://cr.openjdk.java.net/~alanb/8220493/0/webrev/ <http://cr.openjdk.java.net/~alanb/8220493/0/webrev/>I exchanged a few messages offline with Alan regarding review comments
relating to this change. I'm happy to see it move forward and it has my
Review.

Onward, to the new platform SocketImpl and related technical debt
cleanup!

-Chris.
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