face: put the pcap handle in non-blocking mode

This prevents NFD from getting stuck in pcap_next_ex()
when the network interface goes down

Change-Id: I5760fcc7c950b46aecb7619bd889508ee24133aa
diff --git a/daemon/face/ethernet-transport.cpp b/daemon/face/ethernet-transport.cpp
index c122a80..a1c23f2 100644
--- a/daemon/face/ethernet-transport.cpp
+++ b/daemon/face/ethernet-transport.cpp
@@ -167,7 +167,7 @@
 
   auto [pkt, readErr] = m_pcap.readNextPacket();
   if (pkt.empty()) {
-    NFD_LOG_FACE_WARN("Read error: " << readErr);
+    NFD_LOG_FACE_DEBUG("Read error: " << readErr);
   }
   else {
     auto [eh, frameErr] = ethernet::checkFrameHeader(pkt, m_srcAddress,