runtime: switch to deferred STP_EXIT signalling

The former code allowed stapio to have two concurrent commands sent
toward the runtime at the same time: one from the main loop and
another one from the signal handling thread.  This is suspected to
have caused occasional lockups.  This new code switches to a deferred
signal handling model, where the signal handling thread merely
increments a counter, and the main loop monitors that count.
Unfortunately, this currently involves switching the main loop's
blocking /.ctl read into a nonblocking poll.

* runtime/staprun/mainloop.c (pending_interrupts): New variable.
  (signal_thread): Increment it.
  (stp_main_loop): Check it.

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