The signal handler in the isc_loop would wait for all the work to finish
before interrupting the signing. Add teardown handlers via
isc_loopmgr_teardown() to signal the assignwork() it should stop signing
and bail-out early.
NOTE: The dnssec-signzone binary still can't be interrupted during zone
loading, zone cleaning, nsec(3) chain generation or zone writing. This
might get addressed in the future if it becomes a problem.
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