mgmt: use a separate validator for prefix announcements

This commit also configures the default prefix_announcement_validation
section in nfd.conf.sample to accept any prefix announcement.

refs: #5031
Change-Id: I82e34ce783dfe77c170dd02e78a4bc86faa0147a
diff --git a/nfd.conf.sample.in b/nfd.conf.sample.in
index eec2bfc..4cd4e26 100644
--- a/nfd.conf.sample.in
+++ b/nfd.conf.sample.in
@@ -414,6 +414,15 @@
   ;   }
   ; }
 
+  ; The following prefix_announcement_validation accepts any prefix announcement
+  prefix_announcement_validation
+  {
+    trust-anchor
+    {
+      type any
+    }
+  }
+
   auto_prefix_propagate
   {
     cost 15 ; forwarding cost of prefix registered on remote router