Review Request - JDK-4239752: FileSystem should be a platform-specific class to avoid native code

[Dan Xu]

Hi,

Please review the code change to avoid native codes when creating the 
FileSystem object, http://cr.openjdk.java.net/~dxu/4239752/webrev/.

In the change, the native codes for windows and unix platforms are 
removed. Instead, corresponding Java codes are added for each platform. 
Thanks!

-Dan