Allow DFS Links to be placed deeply under DFS Namespace Root

While reading the Microsoft DFS tutorial, I found that DFS links
may be placed in non-targetted folders under the DFS namespace.
For instance: \\server\namespace\folder1\folder2\link.

The code assumed that the links were directly under the namespace,
and this corrects that poor assumption.  However this complicated
WindowsFileDelegate.isDfsLink().  That method is called for every
crawled document where we would be performing a network DFS getInfo
call for every file.

A test of calling isDfsLink() against 250,000 pathnames via a DFS
server linked to a local network file system took 515 millisecs.
So I don't believe calling isDfsLink() for every crawled file will
significantly impact performance.

Code Review: http://codereview.appspot.com/184360043