Clean up libpcap on exit - Brian Russell <brussell@brocade.com>

On exit, call pcap_close() so that any resources used internally
by libpcap can be cleaned up.

http://lists.beasts.org/pipermail/iftop-users/2016-September/000455.html

iftop.c

@@ -839,6 +839,8 @@ int main(int argc, char **argv) {
     }
 
     pthread_cancel(thread);
+    pthread_join(thread, NULL);
+    pcap_close(pd);
 
     ui_finish();