BUG/MEDIUM: kqueue/threads: use one kqueue_fd per thread

BUG/MEDIUM: kqueue/threads: use one kqueue_fd per thread

This is the same principle as the previous patch (BUG/MEDIUM:
epoll/threads: use one epoll_fd per thread) except that this time it's
for kqueue. We don't want all threads to wake up because of activity on
a single other thread that the other ones are not interested in.

Just like with previous patch, this one shows that the polling state
doesn't need to be changed here and that some simplifications are now
possible. This patch only implements the minimum required for a stable
backport.

This should be backported to 1.8.