face: introduce PcapHelper class

Change-Id: I26f7d43394e1b17f314c226ca6bce723c2410ae3
Refs: #4011
diff --git a/daemon/face/unicast-ethernet-transport.cpp b/daemon/face/unicast-ethernet-transport.cpp
index e21a793..eb34648 100644
--- a/daemon/face/unicast-ethernet-transport.cpp
+++ b/daemon/face/unicast-ethernet-transport.cpp
@@ -55,9 +55,9 @@
   snprintf(filter, sizeof(filter),
            "(ether proto 0x%x) && (ether src %s) && (ether dst %s) && (not vlan)",
            ethernet::ETHERTYPE_NDN,
-           m_destAddress.toString().c_str(),
-           m_srcAddress.toString().c_str());
-  setPacketFilter(filter);
+           m_destAddress.toString().data(),
+           m_srcAddress.toString().data());
+  m_pcap.setPacketFilter(filter);
 
   // TODO: implement close on idle and persistency change
 }