[Dovecot] dovecot authentication & exim in rc27

[Richard P. Welty]

[introduction: new to the list, used dovecot w/exim as MTA in
  simple ways for 2 years, blah blah blah]

i'm trying to get exim to use dovecot auth with a copy of
rc27 and am missing something that's probably quite simple.

specifically, how does one configure the authenticators
in dovecot.conf for rc27? the auth section of the default
config file installed in rc27 doesn't entirely look like
what i'm seeing in other documentation; instead of

auth default {

}

i'm seeing

auth = default

followed by lots of standard flags prefixed by auth_,
but i don't see how to set up the socket. i have the
following; dovecot starts and doesn't complain but it
doesn't create the socket either:


auth_socket listen {
   client {
     path = /var/run/dovecot/auth-client
     mode = 0660
     user = exim
     group = exim
   }
}

any suggestions?

thanks,
   richard