Add --print-sampling option to print every Nth packet

[tcpdump] / tests / print-sampling.out

    3  03:57:35.938167 IP 127.0.0.1.55920 > 127.0.0.1.80: Flags [.], ack 930778610, win 8192, options [nop,nop,TS val 1306300950 ecr 1306300950], length 0
    6  03:57:35.941232 IP 127.0.0.1.80 > 127.0.0.1.55920: Flags [P.], seq 930778610:930784169, ack 928549449, win 8192, options [nop,nop,TS val 1306300953 ecr 1306300951], length 5559: HTTP: HTTP/1.1 200 OK
    9  03:57:37.230839 IP 127.0.0.1.80 > 127.0.0.1.55920: Flags [F.], seq 930784169, ack 928549450, win 8192, options [nop,nop,TS val 1306302243 ecr 1306302241], length 0