face: allow setting default UDP face MTU in config

refs #5138

Change-Id: Ibb3767b27aec2b046d1f41292f3d61001866f8c0
diff --git a/daemon/face/udp-factory.hpp b/daemon/face/udp-factory.hpp
index a57cb25..a0f5b6d 100644
--- a/daemon/face/udp-factory.hpp
+++ b/daemon/face/udp-factory.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2014-2018,  Regents of the University of California,
+ * Copyright (c) 2014-2021,  Regents of the University of California,
  *                           Arizona Board of Regents,
  *                           Colorado State University,
  *                           University Pierre & Marie Curie, Sorbonne University,
@@ -123,6 +123,7 @@
 
 private:
   bool m_wantCongestionMarking = false;
+  size_t m_defaultUnicastMtu = ndn::MAX_NDN_PACKET_SIZE;
   std::map<udp::Endpoint, shared_ptr<UdpChannel>> m_channels;
 
   struct MulticastConfig