face: refactor handling of LinkType face trait

This commit also includes some cleanups, fixes EthernetFace to report
the correct LinkType, and makes (get|set)Description non-virtual.

Change-Id: I53909d7bfa02a92641b523a766359413c966aae6
Refs: #2563
diff --git a/daemon/face/channel.hpp b/daemon/face/channel.hpp
index 810d726..ea4b751 100644
--- a/daemon/face/channel.hpp
+++ b/daemon/face/channel.hpp
@@ -25,10 +25,12 @@
 #ifndef NFD_DAEMON_FACE_CHANNEL_HPP
 #define NFD_DAEMON_FACE_CHANNEL_HPP
 
-#include "face.hpp"
+#include "common.hpp"
 
 namespace nfd {
 
+class Face;
+
 class Channel : noncopyable
 {
 public: