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 c100fd2f1fe024cb2f731bfdd283cee36259e6e3)
CVE-2013-2157