mgmt: publish GeneralStatus dataset

In ForwarderStatusManager, publish ForwarderStatus as a dataset at
/localhost/nfd/status/general.
To maintain backwards compatibility, an Interest to /localhost/nfd/status
also retrieves this dataset.

It's intentional not to update the test suite in this commit,
to illustrate that backwards compatibility is maintained.

refs #3081

Change-Id: I9b636e2e621563185303a7304be402c8221c9863
diff --git a/tools/nfd-status.cpp b/tools/nfd-status.cpp
index 1b16c33..f186c8d 100644
--- a/tools/nfd-status.cpp
+++ b/tools/nfd-status.cpp
@@ -162,7 +162,7 @@
   void
   fetchVersionInformation()
   {
-    Interest interest("/localhost/nfd/status");
+    Interest interest("/localhost/nfd/status/general");
     interest.setChildSelector(1);
     interest.setMustBeFresh(true);
     m_face.expressInterest(