(ITS#6665) back_ldap and back_meta chasing referrals authentication

[jsceballos@gmail.com]

Full_Name: Javier Sanz
Version: 2.4.17
OS: Debian Linux 5.0
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (80.38.203.12)


back_ldap and back_meta should be able to do a non-anonymous bind to the
referrals returned by the external LDAP servers. 

This is a regression since 2.3, because the old directives "pseudorootdn" and
"pseudorootpw" allowed specifying the binddn and password that would be used to
chase the referrals, but their 2.4 replacements "idassert-bind" and
"idassert-authzFrom" do not allow that, so these binds are always done
anonymously.