dovecot: fix authed username for system users

dovecot/templates/conf.d/10-auth.conf.j2

@@ -48,7 +48,11 @@ disable_plaintext_auth = yes
 # the standard variables here, eg. %Lu would lowercase the username, %n would
 # drop away the domain if it was given, or "%n-AT-%d" would change the '@' into
 # "-AT-". This translation is done after auth_username_translation changes.
+{% if dovecot_auth_system and not dovecot_auth_virtual %}
+auth_username_format = %Ln
+{% else %}
 #auth_username_format = %Lu
+{% endif %}
 
 # If you want to allow master users to log in by specifying the master
 # username within the normal username string (ie. not using SASL mechanism's