Force simple Bind for authentication

The authentication code was using a common code path with
other LDAP code that got an LDAP connection.  If the system
was configured to do Anonymous binding, users could by pass
the authentication check.

This patch forces the authentication code to do a simple_bind.

Change-Id: Id0c19f09d615446927db1ba074561b129329b5c8
(cherry picked from 6fc4c4e78a6c281505e4c5f542542c8c2cb1f66a)
CVE-2013-2157