samples: pktgen: add trap SIGINT for printing execution result

All pktgen samples can send indefinitely num messages per thread by
setting the count option to 0(-n 0). If running sample with setting
count 0 and press Ctrl-C to stop this program, the program prints the
result of the execution so far. Currently, the samples besides
sample{3...5} don't work properly. Because Ctrl-C stops the script, not
just pktgen.

This is results of samples:

    # DEV=eth0 DEST_IP=10.1.0.1 DST_MAC=00:11:22:33:44:55 ./pktgen_sample04_many_flows.sh -n 0
    Running... ctrl^C to stop
    ^CDevice: eth0@0
    Result: OK: 569657(c569538+d118) usec, 84650 (60byte,0frags)
    148597pps 71Mb/sec (71326560bps) errors: 0

    # DEV=eth0 DEST_IP=10.1.0.1 DST_MAC=00:11:22:33:44:55 ./pktgen_sample01_simple.sh -n 0
    Running... ctrl^C to stop
    ^C

In order to solve this, this commit adds trap SIGINT. Also, this commit
changes control_c function to print_result to maintain consistency with
other samples.

Signed-off-by: Juhee Kang <claudiajkang@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>