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

refs #4005

Change-Id: I01d98b88cdee41b633f6fb9a5600088efe8de749
diff --git a/daemon/face/unicast-udp-transport.hpp b/daemon/face/unicast-udp-transport.hpp
index b1eb672..03f1c86 100644
--- a/daemon/face/unicast-udp-transport.hpp
+++ b/daemon/face/unicast-udp-transport.hpp
@@ -42,7 +42,8 @@
 public:
   UnicastUdpTransport(protocol::socket&& socket,
                       ndn::nfd::FacePersistency persistency,
-                      time::nanoseconds idleTimeout);
+                      time::nanoseconds idleTimeout,
+                      optional<ssize_t> overrideMtu = {});
 
 protected:
   bool