Revert "unix: set O_NONBLOCK in uv_pipe_open()"

It turns out that node.js relies on the blocking behavior of pipes in
some cases, notably when forking worker processes.  Reopens #941.

This reverts commit 8fe4ca686b.

src/unix/pipe.c

@@ -160,9 +160,6 @@ int uv_pipe_open(uv_pipe_t* handle, uv_file fd) {
     return -1;
 #endif /* defined(__APPLE__) */
 
-  if (uv__nonblock(fd, 1))
-    return uv__set_sys_error(handle->loop, errno);
-
   return uv__stream_open((uv_stream_t*)handle,
                          fd,
                          UV_STREAM_READABLE | UV_STREAM_WRITABLE);