In the kqueue backend, do not report EBADF as an EV_READ

In the kqueue backend, do not report EBADF as an EV_READ

We were doing this because of (correct) reports that NetBSD gives an
EBADF when you try to add the write side of a pipe for which the
read side has been closed.  But on most kqueue platforms, that
doesn't happen, and on *all* kqueue platforms, reporting a
nonexistent fd (which we usually have if we have seen EBADF) as
readable tends to give programs a case of the vapors.

Nicholas Marriott wrote the original patch here; I did the comment
fixes.