unix: unconditionally stop handle on close

Make sure the handle is fully stopped by the time uv__stream_close()
calls uv_read_stop(). Fixes the following assertion:

    Assertion failed: (!uv__io_active(&stream->io_watcher, UV__POLLOUT)
    || !ngx_queue_empty(&stream->write_completed_queue)
    || !ngx_queue_empty(&stream->write_queue)
    || stream->shutdown_req != NULL
    || stream->connect_req != NULL), function uv_read_stop,
    file ../deps/uv/src/unix/stream.c, line 1329.

Fixes joyent/node#5622.