Cleanup FileDescriptor implementation

The Unix and Windows versions of FileDescriptor.java could be merged leaving only the native code FileDescriptor_md.c forks for the respective OS.

Additional cleanup might simplify the native code and remove the apparent overlap between FileDescriptor.close0 and FileDescriptor.cleanupClose0.
Note that sockets use the same method to avoid races to close when calling io_util_md.c:fileClose.

The FileDescriptor FDCleanup mechanism can be refactored to improve encapsulation and maintainability as suggested.

http://mail.openjdk.java.net/pipermail/core-libs-dev/2018-February/051253.html