ds: rely on refcounting to close descriptors

ds: rely on refcounting to close descriptors

Since we have EPOLL_CTL_DEL implemented for the poll(2) and
kqueue backends, we can rely on Perl refcounting to gently
close(2) the underlying file descriptors as references get
dropped.

This may be beneficial in the future if we want to drop a
descriptor from the event loop without actually closing it.