Fix b/16619976 Part Deux: Adaptor Feeds improper DFS share ACLs.

From the Properties panel when setting permissions on a DFS link:

“Specify how DFS Namespaces controls which users can see this folder.
 By default, permissions are inherited from local file system of the
 namespace server, granting all domain users the ability to view all
 folders in the namespace.”

This change replaces the call to NetDfsGetSecurity() with a call
to GetFileSecurity() on the namespace when there is no explicit
ACL on the DFS link.

Code Review:  http://codereview.appspot.com/191130043