This new mode makes it easy to run Pageant as a "supervised" instance,
e.g. as part of a test harness for other programs interacting with an
SSH agent, which is the original use case. Because Pageant is then
running as a child process of the supervisor, the operating system
notifies the supervisor of the child's aliveness without resorting to
PIDs or socket addresses, both of which may principally run stale and/or
get recycled.
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