[Dovecot] multiple passdbs and auth sockets

[Marco d'Itri]

I have a few systems which run dovecot 1.x as the authentication backend 
for Postfix, with multiple auth { } sections like this one, each one 
with a different passdb and its own socket:

auth ldap-10 {
  passdb ldap {
    args = /etc/dovecot/dovecot-ldap-10.conf
   }
  socket listen {
    client {
      path = /var/spool/postfix/private/auth-10
      mode = 0666
    }
  }
}

I need multiple sockets tied to multiple dbs because there are duplicated 
accounts, but after switching to dovecot 2.1.7 apparently *all* passdbs 
are checked and dovecot reports this message:

Warning: Obsolete setting in /etc/dovecot/conf.d/auth-local.conf.ext:91: add auth_ prefix to all settings inside auth {} and remove the auth {} section completely

doveconf shows that all my custom sockets are now part of a single 
"service auth { }" section, while all the passdbs appear at the top 
level.

How can I update my configuration for dovecot 2.x?

-- 
ciao,
Marco