try to avoid kill -9 for stopping nodes

Using kill -9 on a node can leave it in a zombie process state, stuck in a
system call never to return. OS X Mavericks seems especially vunerable to
this problem. Only a reboot can clear out such zombies. Change
rt:brutal_kill/1 to try a normal kill -15 first, and set a 5 second timer
to perform a kill -9 if the normal kill doesn't work. Change the kill -9 in
rtdev:stop_all/1 to a normal kill -15.