GetQueuedCompletionStatus() document on MSDN says the
following signature:

    http://msdn.microsoft.com/en-us/library/windows/desktop/aa364986.aspx

    BOOL WINAPI GetQueuedCompletionStatus(
      _In_   HANDLE CompletionPort,
      _Out_  LPDWORD lpNumberOfBytes,
      _Out_  PULONG_PTR lpCompletionKey,
      _Out_  LPOVERLAPPED *lpOverlapped,
      _In_   DWORD dwMilliseconds
    );

In the latest specification, the type of the third argument
(lpCompletionKey) is PULONG_PTR not LPDWORD.

In theory, this can provide a bit better distribution of latencies.

Also it simplifies the code, since ngx_queue_t is now used instead
of custom implementation.

It's mostly dead code.  And the idea of thread support for this task has
been deprecated.

This fixes --with-file-aio support on systems that lack eventfd()
syscall, notably aarch64 Linux.

The syscall(SYS_eventfd) may still be necessary on systems that
have eventfd() syscall in the kernel but lack it in glibc, e.g.
as seen in the current CentOS 5 release.

Since Linux 2.6.17, epoll is able to report about peer half-closed connection
using special EPOLLRDHUP flag on a read event.

Several warnings silenced, notably (ngx_socket_t) -1 is now checked
on socket operations instead of -1, as ngx_socket_t is unsigned on win32
and gcc complains on comparison.

With this patch, it's now possible to compile nginx using mingw gcc,
with options we normally compile on win32.