pipe: allow queueing pending handles

Introduce `int uv_pipe_pending_count(uv_pipe_t*)` and
`uv_handle_type uv_pipe_pending_type(uv_pipe_t*)`. They should be
used in IPC pipe's read cb to accept incoming handles:

    int count = uv_pipe_pending_count(pipe);
    int i;
    for (i = 0; i < count; i++) {
      uv_handle_type type = uv_pipe_pending_type(pipe);
      /* ... */
      uv_accept(...);
    }