Make the kqueue ioset back-end less whiny.

Make the kqueue ioset back-end less whiny.

src/ioset-kevent.c (ioset_kevent_add): Never try to use EV_DELETE, because
    that can make kevent() return -1/ENOENT.
  (ioset_kevent_update): We need to try to EV_DELETE here instead.
  (ioset_kevent_loop): Do not bail if kevent() just returns -1/EINTR.