More option for static probe generation

Static probes can be generated with the following option to tracebox:
 -6 enable IPv6 probe (default is IP).
 -u enable UDP probe (default is TCP).
 -d <port> uses port as destination port (default is 80).

Examples:
 - a UDP/DNS probe to 8.8.8.8 (not that no payload is added):
  $ tracebox -i eth0 -u -d 53 8.8.8.8
 - a IPv6/SSH probe to <dest>
  $ tracebox -i eth0 -6 -d 22 <dest>

Signed-off-by: Gregory Detal <gregory.detal@uclouvain.be>
Detect default iface if not specified

Signed-off-by: Gregory Detal <gregory.detal@uclouvain.be>