poll needs similar treatment as select recently gained in order to not

poll needs similar treatment as select recently gained in order to not
mask failed kqueue_register() attempts which in turn causes the thread to
end up sleeping as opposed of propagating the failure to user space.

Regression introduced in the recent switch to implement poll on top of
kqueue.

Thanks to Larry Hynes <larry at larryhynes dot com> for reporting and
Leah Neukirchen <leah at vuxu dot org> for providing a reproducer; which
also already made its way into regress.

ok mpi@