nixos/tsm-client: Fix multi-value dsm.sys options

A configuration such as:

    programs.tsmClient.servers.backup.domain = [ "/dir1" "dir2" ];

...would previously result in an error ("cannot coerce a list to a
string"), since `makeDsmSysLines` would return a nested list.

nixos/modules/programs/tsm-client.nix

@@ -4,7 +4,7 @@ let optionsGlobal = options; in
 let
 
   inherit (lib.attrsets) attrNames attrValues mapAttrsToList removeAttrs;
-  inherit (lib.lists) all allUnique concatLists elem isList map;
+  inherit (lib.lists) all allUnique concatLists concatMap elem isList map;
   inherit (lib.modules) mkDefault mkIf;
   inherit (lib.options) mkEnableOption mkOption mkPackageOption;
   inherit (lib.strings) concatLines match optionalString toLower;
@@ -231,7 +231,7 @@ let
     # Turn a key-value pair from the server options attrset
     # into zero (value==null), one (scalar value) or
     # more (value is list) configuration stanza lines.
-    if isList value then map (makeDsmSysLines key) value else  # recurse into list
+    if isList value then concatMap (makeDsmSysLines key) value else  # recurse into list
     if value == null then [ ] else  # skip `null` value
     [ ("  ${key}${
       if value == true then "" else  # just output key if value is `true`