unix: fix SIGCHLD waitpid() race in process.c

Before this commit, multiple event loops raced with each other when a
SIGCHLD signal was received.  More concretely, it was possible for
event loop A to consume waitpid() events that should have been
delivered to event loop B.

This commit addresses that by doing a linear scan over the list of
child processes.  An O(n) scan is not terribly efficient but the
actual performance impact is not measurable in a benchmark that spawns
rounds of several thousands instances of /bin/false.  For the time
being, this patch will suffice; we can always revisit it later.

Fixes #887.