mgmt: add certificate generation/export instructions to nfd.conf.sample.in and README.md

rename configuration file's "keyfile" field to "certfile" to reflect usage of
  an NDN certificate
nfd.conf certfile paths are now interpreted as relative to nfd.conf's location

refs: #1332

Change-Id: Ib91cffd3d113ef084bf19e87a85172ddfd16b7eb
diff --git a/daemon/mgmt/face-manager.hpp b/daemon/mgmt/face-manager.hpp
index 4139152..8efb509 100644
--- a/daemon/mgmt/face-manager.hpp
+++ b/daemon/mgmt/face-manager.hpp
@@ -83,7 +83,7 @@
 
 private:
   void
-  onConfig(const ConfigSection& configSection, bool isDryRun);
+  onConfig(const ConfigSection& configSection, bool isDryRun, const std::string& filename);
 
   void
   processSectionUnix(const ConfigSection& configSection, bool isDryRun);