network: Handle carrier information

IFLA_CARRIER allows to know when a cable is plugged to the Ethernet
card or when the WiFi is connected. If there's no carrier, the
interface will be considered disconnected.

include/modules/network.hpp

@@ -64,6 +64,7 @@ class Network : public ALabel {
 
   std::string state_;
   std::string essid_;
+  bool        carrier_;
   std::string ifname_;
   std::string ipaddr_;
   std::string netmask_;