fix kqueue EV_SET bug

[zt2]

Hi,

I find using Net::HTTP will hangs forever with kqueue on mac like issue #6 , for example:

require 'evt'
require 'net/http'

scheduler = Evt::Scheduler.new

Fiber.set_scheduler scheduler

100.times do
  Fiber.schedule do
    p Net::HTTP.get('httpbin.org', '/ip')
  end
end

After some researching, I found this problem is trigged in ext/evt/kqueue.h:

    if (ruby_interest & readable) {
        event_flags |= EVFILT_READ;
    }

    if (ruby_interest & writable) {
        event_flags |= EVFILT_WRITE;
    }

    EV_SET(&event, fd, event_flags, EV_ADD|EV_ENABLE|EV_ONESHOT, 0, 0, (void*) io);

EVFILT_READ == ffffffff
EVFILT_WRITE == fffffffe

That makes EVFILT_READ | EVFILT_WRITE == EVFILT_READ and the script will hang on one event.

The result is that "or"-ing them together makes kqueue wait for READ events only. Flags can be "or"-ed but filters should not.

So this patch tries to create two events and this problem is solved.