Forward SIGCHLD to the io-waiter.

Forward SIGCHLD to the io-waiter.

Set/clear the new variable smp_io_blocked around calls to poll/select
so that subprocess_death (SIGCHLD) can wake a blocked processor
without having to ask Scheme which (if any) is running the io-waiter.
Wake the processor with a timer interrupt withOUT fiddling next_timer.