* Retrieve the hostname of the server for this SMB resource. If this
     * <code>SmbFile</code> references a workgroup, the name of the workgroup
     * is returned. If this <code>SmbFile</code> refers to the root of this
     * SMB network hierarchy, <code>null</code> is returned.
     * 
     * @return The server or workgroup name or <code>null</code> if this
     *         <code>SmbFile</code> refers to the root <code>smb://</code> resource.