Add the directive AuthLDAPAllowDNAuth to allow a user to authenticate against...

Add the directive AuthLDAPAllowDNAuth to allow a user to authenticate against an LDAP directory using a full user DN.  This directive allows a user to authenticate against a subcontext that may contain non-unique user IDs.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@168016 13f79535-47bb-0310-9956-ffa450edef68