runqueue: Improve sigchld handler

The sigchld handler was reaping any processes and this was leading to
confusion with any other process handling code that could be active.
This patch:

a) Ensures we only read any process results for the worker processes
   we want to monitor
b) Ensures we pass the event to any other sigchld handler if
   it isn't an event we're interested in so the functions are properly
   chained.

Together this should resolve some of the reports of unknown processes
people have been reporting.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>