Set mailserver.policydSPFExtraConfig in a debug module

The line type of this option make its concatenation cleaner: the user
doesn't have to manually add `\n` to its policydSPFExtraConfig value
when debug in set.

mail-server/debug.nix

@@ -0,0 +1,4 @@
+{ config, lib, ... }:
+{
+  mailserver.policydSPFExtraConfig = lib.mkIf config.mailserver.debug "debugLevel = 4";
+}

mail-server/postfix.nix

@@ -114,11 +114,7 @@ let
    (lib.optional cfg.dkimSigning "unix:/run/opendkim/opendkim.sock")
    ++ [ "unix:/run/rspamd/rspamd-milter.sock" ];
 
-  policyd-spf = pkgs.writeText "policyd-spf.conf" (
-    cfg.policydSPFExtraConfig
-    + (lib.optionalString cfg.debug ''
-    debugLevel = 4
-  ''));
+  policyd-spf = pkgs.writeText "policyd-spf.conf" cfg.policydSPFExtraConfig;
 
   mappedFile = name: "hash:/var/lib/postfix/conf/${name}";