nixos/openldap: switch to slapd.d configuration

The old slapd.conf is deprecated. Replace with slapd.d, and use this
opportunity to write some structured settings.

Incidentally, this fixes the fact that openldap is reported up before
any checks have completed, by using forking mode.

nixos/modules/misc/ids.nix

@@ -135,7 +135,7 @@ in
       #keys = 96; # unused
       #haproxy = 97; # dynamically allocated as of 2020-03-11
       mongodb = 98;
-      openldap = 99;
+      #openldap = 99; # dynamically allocated as of PR#94610
       #users = 100; # unused
       cgminer = 101;
       munin = 102;
@@ -451,7 +451,7 @@ in
       keys = 96;
       #haproxy = 97; # dynamically allocated as of 2020-03-11
       #mongodb = 98; # unused
-      openldap = 99;
+      #openldap = 99; # dynamically allocated as of PR#94610
       munin = 102;
       #logcheck = 103; # unused
       #nix-ssh = 104; # unused