Duration and Graceful stop

[s8sg]

Added option to provide duration -d

./dnsflood test.com 127.0.0.1 -d 30
sent 5333186 DNS requests in 30.000000 sec

Performing graceful stop on SIGINT and SIGTERM

./dnsflood test.com 127.0.0.1 -d 30
^CStopping..
sent 305814 DNS requests in 2.000000 sec