Directly cause assertion failure on pthreads primitives failure

Instead of returning error values from isc_rwlock_*(), isc_mutex_*(),
and isc_condition_*() macros/functions and subsequently carrying out
runtime assertion checks on the return values in the calling code,
trigger assertion failures directly in those macros/functions whenever
any pthread function returns an error, as there is no point in
continuing execution in such a case anyway.

lib/isc/timer.c

@@ -570,7 +570,7 @@ isc__timermgr_destroy(isc_timermgr_t **managerp) {
 	/*
 	 * Clean up.
 	 */
-	(void)isc_condition_destroy(&manager->wakeup);
+	isc_condition_destroy(&manager->wakeup);
 	isc_mutex_destroy(&manager->lock);
 	isc_heap_destroy(&manager->heap);
 	manager->magic = 0;