pping: Add graceful shutdown on SIGTERM

Also intercept SIGTERM (in addition the the previously intercepted
SIGINT) and perform graceful shutdown.

Perhaps it also makes sense to perform graceful shutdown on some
additional signals, like SIGHUP and SIGQUIT?

Signed-off-by: Simon Sundberg <simon.sundberg@kau.se>

pping/pping.c

@@ -1024,6 +1024,7 @@ int main(int argc, char *argv[])
 
 	// Allow program to perform cleanup on Ctrl-C
 	signal(SIGINT, abort_program);
+	signal(SIGTERM, abort_program);
 
 	// Main loop
 	while (keep_running) {