Implemented EV_CLOSED event for kqueue backend.

- Implemented EV_CLOSED event for kqueue. Detects premature connection close by clients
  without the necessity of reading all the pending data. Does not depend on EV_READ and/or EV_WRITE.

- Added new EV_FEATURE_EARLY_CLOSED feature for kqueue. Must be supported for listening to EV_CLOSED event.

- Existing regression test now working for kqueue: test-closed.c

- All regression tests passed (test/regress and test/test.sh). Tested on OS X Mavericks and FreeBSD 9.0

- FreeBSD truss output:
  ```
  kqueue(0x801021070,0x0,0x0,0x0,0x30,0x7fffffffd010) = 3 (0x3)
  kevent(3,{0xffffffffffffffff,EVFILT_READ,EV_ADD,0,0x0,0x0},1,
           {0xffffffffffffffff,EVFILT_READ,EV_ERROR,0,0x9,0x0},
            64,0x0) = 1 (0x1)
  socketpair(0x1,0x1,0x0,0x7fffffffda70,0x10,0x7fffffffd010) = 0 (0x0)
  sendto(4,"test string\0",12,0x0,NULL,0x0)    = 12 (0xc)
  shutdown(4,SHUT_WR)              = 0 (0x0)
  kevent(3,{0x5,EVFILT_READ,EV_ADD,0,0x0,0x8000},1,
           {0x5,EVFILT_READ,EV_EOF,0,0xc,0x8000},
           64,{2.999297000 }) = 1 (0x1)
  write(1,"closed_cb: detected socket close"...,46) = 46 (0x2e)
  ```