RFR 8058824: Drop TwoStacks socket implementation in jdk9 [win] - Part I

[Ivan Gerasimov]

Hello!

Here is a proposal for the first step in cleaning up the TwoStacks 
socket implementation.
It is proposed to drom IPv6 support in the implementation of 
TwoStacksPlainSocketImpl class (so formally, it's no more Two Stacks).
Therefore, this class will only be used when the user explicitly 
requests IPv4 only configuration through setting 
java.net.preferIPv4Stack option.

The fix mostly consists of removal, so it should be relatively easy to 
review.

All the jdk_net tests pass.

BUGURL: https://bugs.openjdk.java.net/browse/JDK-8058824
WEBREV: http://cr.openjdk.java.net/~igerasim/8058824/0/webrev/

Sincerely yours,
Ivan