(ITS#6665) back_ldap and back_meta chasing referrals authentication

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.