mod_ldap: "LDAPReferrals off" does not disable LDAPReferrals feature.

Make "off" really "off" and add "unset" to take SDK defaults

trunk patch: http://svn.apache.org/r1517388
       docs: http://svn.apache.org/r1518265

Submitted by: covener
Reviewed by: jim, humbedooh


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1523263 13f79535-47bb-0310-9956-ffa450edef68