RFR [8058099] (ch) Cleanup in FileChannel/FileDispatcher native implementation [win]

[Ivan Gerasimov]

Hello!

This is a proposal to slightly rework FileChannel/FileDispatcher native 
implementation on Windows.
If the newer API is used (which is already available on XP and 2003), 
this will make code a bit clear and let us save a few redundant calls.

Would you please help review the change?

BUGURL: https://bugs.openjdk.java.net/browse/JDK-8058099
WEBREV: http://cr.openjdk.java.net/~igerasim/8058099/0/webrev/

Sincerely yours,
Ivan