JNA wrapper for <sys/xattr.h>

Here is my wrapper for <sys/xattr.h> with support for xattr list/get/set/remove.

Tested on OS X 10.9.2
`Pointer.setString()` *does* NUL-terminate, but since you need to allocate space here as well, that function doesn't help much.  The class `NativeString` actually does what you want, but it's package-protected.

I think your code is fine as is.