Fix dns_matching example

* The name encoding function was not checking the constraints on domain
  names properly (<= 253 chars as one byte is needed for the name of the
  label and one for the terminating 0-len label; <= 63 chars per label).
* The name encoding function was erroring when assigning a struct value
  in the byte array (Python 3.6.3). Refactored to join successive
  subarrays, and moved the null padding to make it explicit that it is
  needed by the bpf map key (and not the dns encoding).
* Used builtin from argparse to have a list of domains in the command
  line arguments.
* Reset the non-block flag through fcntl directly instead of
  reconstructing a socket object.
* Exit gracefully when triggering SIGINT as hinted.