mgmt+face: allow MTU of datagram faces to be overridden

refs #4005

Change-Id: I01d98b88cdee41b633f6fb9a5600088efe8de749
diff --git a/daemon/face/channel.hpp b/daemon/face/channel.hpp
index db97cc8..6d2c221 100644
--- a/daemon/face/channel.hpp
+++ b/daemon/face/channel.hpp
@@ -89,6 +89,7 @@
   ndn::nfd::FacePersistency persistency = ndn::nfd::FACE_PERSISTENCY_PERSISTENT;
   optional<time::nanoseconds> baseCongestionMarkingInterval;
   optional<uint64_t> defaultCongestionThreshold;
+  optional<ssize_t> mtu;
   bool wantLocalFields = false;
   bool wantLpReliability = false;
   boost::logic::tribool wantCongestionMarking = boost::logic::indeterminate;