libports: Force threads to wake up periodically

Quiescence support in port-deref-deferred.c assumes that all threads will
sooner or later go through a quiescent state (because it finished processing
a message). But that is not true: proc doesn't set a thread timeout, and
thus some threads can stay indefinitely stuck in receiving messages. And
thus the deferred dereferencing used by ports_destroy_right never gets
achieved. This accumulation can be seen by running:

while true ; do echo $(echo -n $(echo a)) > /dev/null ; done

while running vminfo 4 | wc -l in parallel.

Making threads get out of mach_msg at least periodically allows
unstucking quiescence generations.

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