Create sniffer

/* basic packet sniffer
 *
 * this takes one argument, an interface name, and starts packet sniffing on it
 * it prints out only src and dst mac, and the field where either the ethertype or vlan type goes
 * modified from http://www.binarytides.com/packet-sniffer-code-in-c-using-linux-sockets-bsd-part-2/
 */