add semaphore to synchronize packet sending and receiving action for arp.c, and exit if no arp reply was sent in 2 seconds.
rewrite impersonate.c, just sending meaningless ethernet packet to impersonate and check every 1 minute if the mac address of the victim has been changed, update if so.
add leave_promiscuous function to leave promiscuous mode for net_common.c, fixed network interface flag operation.