data: Optimize Data signing

As of this commit, KeyChain::sign pre-allocates EncodingBuffer, requests
unsigned portion of Data using Data::wireEncode(EncodingBuffer, true),
and then appends the resulting signature and prepends Data packet
header.  This way there is no extra memory allocation after Data packet
is signed.

Change-Id: I670e9a2f1d6f5e9b049f41b47f011af384f32c95
diff --git a/src/management/nfd-face-flags.hpp b/src/management/nfd-face-flags.hpp
index d7f8a14..39c85cb 100644
--- a/src/management/nfd-face-flags.hpp
+++ b/src/management/nfd-face-flags.hpp
@@ -27,10 +27,9 @@
 
 /**
  * \ingroup management
- * \enum FaceFlags
  * \brief provides additional information about a face
  */
-enum {
+enum FaceFlags {
   /** \brief face is local (for scope control purpose)
    */
   FACE_IS_LOCAL = 1,