fix: include timeout as part of the generators/wait conversation

fix: include timeout as part of the generators/wait conversation

So far, the wait functions would have shielded the generators from a
wait timeout. However this behaviour makes impossible to make a
generator interruptible.

Note that the `wait_c` generator was interruptible, but probably it
wasn't doing the right thing. In the `poll` branch, I understand that
the returned ready value, in case of timeout, would have been the same
of the input wait value, because of the input/output nature of the
pollfd struct; I haven't analyzed more deeply the select() case.