* Returns the <tt>NtlmPasswordAuthentication</tt> object used as
 * credentials with this file or pipe. This can be used to retrieve the
 * username for example:
 * <tt>
 * String username = f.getPrincipal().getName();
 * </tt>
 * The <tt>Principal</tt> object returned will never be <tt>null</tt>
 * however the username can be <tt>null</tt> indication anonymous
 * credentials were used (e.g. some IPC$ services).
 */