stapio main loop: always time out on pselect()

stapio main loop: always time out on pselect()

The main thread's pselect() call must now get a timeout setting too,
because it is responsible for orderly shutdown of the stap module in
case of interrupts.  Without a polling timeout, the pselect() can just
sit there, with other reader threads happily continuing their work,
as though the ^C never took effect.