ssh service: add sftpFlags option

nixos/modules/services/networking/ssh/sshd.nix

···
         '';
       };
 
+      sftpFlags = mkOption {
+        type = with types; listOf str;
+        default = [];
+        example = [ "-f AUTHPRIV" "-l INFO" ];
+        description = ''
+          Commandline flags to add to sftp-server.
+        '';
+      };
+
       permitRootLogin = mkOption {
         default = "prohibit-password";
         type = types.enum ["yes" "without-password" "prohibit-password" "forced-commands-only" "no"];
···
       };
 
       moduliFile = mkOption {
-        example = "services.openssh.moduliFile = /etc/my-local-ssh-moduli;";
+        example = "/etc/my-local-ssh-moduli;";
         type = types.path;
         description = ''
           Path to <literal>moduli</literal> file to install in
···
         ''}
 
         ${optionalString cfg.allowSFTP ''
-          Subsystem sftp ${cfgc.package}/libexec/sftp-server
+          Subsystem sftp ${cfgc.package}/libexec/sftp-server ${concatStringsSep " " cfg.sftpFlags}
         ''}
 
         PermitRootLogin ${cfg.permitRootLogin}