Allow interrupting dnssec-signzone during signing

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.

bin/dnssec/dnssec-signzone.c

@@ -1544,6 +1544,13 @@ signapex(void) {
 	}
 }
 
+static void
+abortwork(void *arg) {
+	UNUSED(arg);
+
+	atomic_store(&shuttingdown, true);
+}
+
 /*%
  * Assigns a node to a worker thread.  This is protected by the main task's
  * lock.
@@ -3937,13 +3944,13 @@ main(int argc, char *argv[]) {
 		 * processors if possible.
 		 */
 		isc_loopmgr_setup(loopmgr, assignwork, NULL);
+		isc_loopmgr_teardown(loopmgr, abortwork, NULL);
 		isc_loopmgr_run(loopmgr);
 
 		if (!atomic_load(&finished)) {
 			fatal("process aborted by user");
 		}
 	}
-	atomic_store(&shuttingdown, true);
 	postsign();
 	TIME_NOW(&sign_finish);