Refactor and modernize namespace declarations

Move all unit tests to namespace nfd::tests

Delete unused header core/algorithm.hpp

Change-Id: I5591f0c5f3bb5db67f8b45fae95471f8a555ca68
diff --git a/daemon/face/pcap-helper.cpp b/daemon/face/pcap-helper.cpp
index d481ac5..054eda2 100644
--- a/daemon/face/pcap-helper.cpp
+++ b/daemon/face/pcap-helper.cpp
@@ -35,8 +35,7 @@
 #define PCAP_NETMASK_UNKNOWN  0xffffffff
 #endif
 
-namespace nfd {
-namespace face {
+namespace nfd::face {
 
 PcapHelper::PcapHelper(const std::string& interfaceName)
   : m_pcap(nullptr)
@@ -143,5 +142,4 @@
     return {{packet, header->caplen}, ""};
 }
 
-} // namespace face
-} // namespace nfd
+} // namespace nfd::face