MEDIUM: poll: don't use the old FD state anymore

MEDIUM: poll: don't use the old FD state anymore

The polling updates are now performed exactly like the epoll/kqueue
ones : only the new polled state is considered, and the previous one
is checked using polled_mask. The only specific stuff here is that
the fd state is shared between all threads, so an FD removal has to
be done only once.