tools: Fix issues with ndns-dig

- Handle the case when validator.conf is not present and anchors cannot
  be found (default path for anchor lookups is $SYSCONFDIR/ndns)

- Abort ndns-dig when positional parameters are omitted

refs: #2276

Change-Id: Id70d4b219254f5b221990a38d41471b920aa1521
diff --git a/src/validator.cpp b/src/validator.cpp
index 8bd4a4a..c3dff78 100644
--- a/src/validator.cpp
+++ b/src/validator.cpp
@@ -70,7 +70,7 @@
       "  file-name \""
       ;
 
-    config += "anchors/root.cert";
+    config += DEFAULT_CONFIG_PATH "/" "anchors/root.cert";
 
     config +=
       "\"                                                                         \n"