The current WaitForMultipleObjects-based implementation causes a
deadlock if 16 or more processes are spawned, as explained in the bug.
This patch replaces the use of events and WaitForMultipleObjects with
I/O Completion Ports (IOCP) as the mechanism to monitor I/O progress
and process terminations. This is not subject to the
MAXIMUM_WAIT_OBJECTS (64) limit of WaitForMultipleObjects.
Original Revision: https://phabricator.services.mozilla.com/D258968
Differential Revision: https://phabricator.services.mozilla.com/D266337
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