use ioctl(SO_TIMESTAMP) for packet timestamp

[nbareil]

net2pcap shall use this ioctl instead of gettimeofday():

socket(7):

   SO_TIMESTAMP
          Enable or disable the receiving of the SO_TIMESTAMP control mes‐
          sage.    The  timestamp  control  message  is  sent  with  level
          SOL_SOCKET and the cmsg_data field is a struct timeval  indicat‐
          ing  the reception time of the last packet passed to the user in
          this call.  See cmsg(3) for details on control messages.