mgmt: report face MTU in faces/list and faces/query datasets

This commit also contains an overall cleanup of FaceManager
and extends the FaceDataset test case.

Change-Id: I8c8290b0dc04b25582e66a5c6dad3cca4dd226eb
Refs: #4763, #3325
diff --git a/tests/daemon/mgmt/fib-manager.t.cpp b/tests/daemon/mgmt/fib-manager.t.cpp
index e2986bc..f0a91ff 100644
--- a/tests/daemon/mgmt/fib-manager.t.cpp
+++ b/tests/daemon/mgmt/fib-manager.t.cpp
@@ -414,7 +414,7 @@
     fibEntry->addNextHop(*m_faceTable.get(addFace()), std::numeric_limits<uint8_t>::max() - 2);
   }
 
-  receiveInterest(Interest("/localhost/nfd/fib/list"));
+  receiveInterest(Interest("/localhost/nfd/fib/list").setCanBePrefix(true));
 
   Block content = concatenateResponses();
   content.parse();