RFR 8202091 : Rename DualStackPlainSocketImpl to PlainSocketImpl [win]

[Ivan Gerasimov]

Hello!

After integrating the fix for JDK-8201510, there is only one 
implementation of PlainSocketImpl left on Windows.

It is proposed to just rename DualStackPlainSocketImpl to 
PlainSocketImpl and avoid the indirection.

Some minor cleanup was done with this change:
- dropping static PlainSocketImpl.isReusePortAvailable() -- it wasn't 
called anyway and just hid the same named function in the super class,
- minor re-formatting.

Would you please help review the fix?

BUGURL: https://bugs.openjdk.java.net/browse/JDK-8194534
WEBREV: http://javaweb.us.oracle.com/~igerasim/webrevs/8194534/00/webrev/

Thanks in advance!

-- 
With kind regards,
Ivan Gerasimov