rib: remote prefix registration

resolve 3 bugs:

There are redundant registrations/unregistrations if loading the
config file multiple times.

Remote registration/unregistration will fail if localhop_security
is enabled.

Unstable RemoteRegistrator/UnregisterAdvanced test case.

Change-Id: I4437292e9f6c0e340c761ef7556a9bdc703ac06c
refs: #2294
diff --git a/nfd.conf.sample.in b/nfd.conf.sample.in
index dafc88a..7edb379 100644
--- a/nfd.conf.sample.in
+++ b/nfd.conf.sample.in
@@ -288,6 +288,16 @@
   ;   ; }
   ; }
 
+  ; The following localhop_security should be enabled when NFD runs on a hub,
+  ; which accepts all remote registrations and is a short-term solution.
+  ; localhop_security
+  ; {
+  ;   trust-anchor
+  ;   {
+  ;     type any
+  ;   }
+  ; }
+
   remote_register
   {
     cost 15 ; forwarding cost of prefix registered on remote router