JCLOUDS-658: use xattr only on supported systems

Mac OS X HFS+ does not support UserDefinedFileAttributeView:

https://bugs.openjdk.java.net/browse/JDK-8030048
@demobox This is the documented way to identify [Mac OS from Java](https://developer.apple.com/library/mac/technotes/tn2002/tn2110.html).