Implemented EV_CLOSED event for kqueue backend.

- Implemented EV_CLOSED event for kqueue.
  Detects premature connection close by clients while
  reading without the necessity of receiving all the pending
  data. Works only with EV_READ, eg: EV_READ | EV_CLOSED.

- Added new EV_FEATURE_EARLY_CLOSED_ON_READ feature for kqueue.
  Must be supported for listening to EV_CLOSED event.

- Added new regression test for kqueue.
  test-closed-on-read.c

- All regression tests passed (test/regress and test/test.sh)
  Tested on OS X Mavericks and FreeBSD 10.0