RFR 8189330 Cleanup FileDescriptor implementation

[Roger Riggs]

Please review a cleanup and refactoring of FileDescriptor.java.
The Unix and Windows versions are merged to ease maintenance remove 
replicated code
The FileCleanable class is extracted and SocketCleanable is updated.

Webrev:
   http://cr.openjdk.java.net/~rriggs/webrev-fd-cleanup-8189330/

Issue:
   https://bugs.openjdk.java.net/browse/JDK-8189330

Thanks, Roger