Make event_loop_stop volatile for safer access

As suggested by Christian Seiler:
"Only minor thing is that you might want to mark
static int event_loop_stop; (usr/event_poll.c)
to be volatile, to be on the safe side when modifying it
from within a signal handler. Probably not really required
here (the compiler is not allowed to optimize out the
access anyway, since you call non-static functions within
the loop), but it doesn't hurt either, just in case... "

usr/event_poll.c

@@ -123,7 +123,7 @@ static int shutdown_wait_pids(void)
 #define POLL_ALARM	2
 #define POLL_MAX	3
 
-static int event_loop_stop;
+static volatile int event_loop_stop;
 static queue_task_t *shutdown_qtask; 
 
 void event_loop_exit(queue_task_t *qtask)