client.c: supervisor task should use post_select instead of pre_select.

This allows to get rid of the ugly min_delay hack. This works
because the supervisor task is registered _after_ the client
task. So when the client task changes its state to either
CL_RECEIVING or CL_SENDING in its post_select function, the
supervisor task will start the stdin/stout task in the same
iteration of the scheduler post_select loop.