director: Cleanup for director socket type configuration.

director: Cleanup for director socket type configuration.
It's now possible to use any type of a socket for inet listeners by
specifying the name for the listener. The available types are: auth
(default), userdb, ring (= director<->director connection), admin/doveadm.
This change should be backwards compatible with previous configuration.
This setting also deprecates director_doveadm_port setting.