arping: Fix 1s delay on exit for unsolicited arpings

Commit 67e070d introduced 1s poll() delay for unsolicited arpings,
when using -U or -A, e.g.:

    # arping -A -c1 -I eth0 [IP address of eth0 interface]
    # arping -U -c1 -I eth0 [IP address of eth0 interface]

Restore correct condition.

Fixes: 67e070d ("arping: use signalfd() and timerfd() rather than signals")
Fixes: iputils#536
Closes: iputils#541
Reported-by: <Curtis Taylor cjebpub@gmail.com>
Suggested-by: <Curtis Taylor cjebpub@gmail.com>
Signed-off-by: Petr Vorel <pvorel@suse.cz>

arping.c

@@ -776,7 +776,7 @@ static int event_loop(struct run_state *ctl)
 	pfds[POLLFD_SOCKET].events = POLLIN | POLLERR | POLLHUP;
 	send_pack(ctl);
 
-	while (!exit_loop) {
+	while (!(exit_loop || ctl->unsolicited)) {
 		int ret;
 		size_t i;