Use while true instead of waiting for a keyboard input to quit

The function is even worse than just waiting for a keyboard press. The
hard-coded assumption in this lwip_unix_keypressed is that select for
fd 1 will only return on a keyboard but sometimes select just returns
without a timeout or for other reasons and then lwipovpn will just
quit at a random point.

app/unixaf_app.c

@@ -479,7 +479,7 @@ main_loop(void) {
 #endif
 
   /* MAIN LOOP for driver update (and timers if NO_SYS) */
-  while (!LWIP_EXAMPLE_APP_ABORT()) {
+  while (true) {
     unixafif_poll(netif_default);
 
 #if ENABLE_LOOPBACK && !LWIP_NETIF_LOOPBACK_MULTITHREADING