by Lwt_unix.wait_read (in read_console, called asynchronously in a fresh task)
and only then convert to a Lwt_io.t (of_fd ~mode:Input) -- thanks to @cfcs
This allows albatross to be run on Linux, where the former code lead to:
- open FIFO
- read <- EOF (although no writer was connected yet)
this works nicely around ocsigen/lwt#741