core: reimplement logging using ndn-cxx's facility

Change-Id: Ifc7c5d70a61ad405dc1f1adfa522a2c0ad1586ab
Refs: #4580
diff --git a/nfd.conf.sample.in b/nfd.conf.sample.in
index c5bf79f..8efc2ab 100644
--- a/nfd.conf.sample.in
+++ b/nfd.conf.sample.in
@@ -33,7 +33,8 @@
   ; Run:
   ;   nfd --modules
   ;
-  ; Or look for NFD_LOG_INIT(<module name>) statements in .cpp files
+  ; Or look for NFD_LOG_INIT(<module name>) statements in source files.
+  ; Note that the "nfd." prefix can be omitted.
   ;
   ; Example module-level settings:
   ;