Terminate child "threads" with _exit() instead of exec(). #4332

We don't want processes created via Create_Thread() to execute C++
destructors when they exit. Both _exit() and exec("/bin/{true|false}")
can accomplish this. Using exec() means we only get a binary result of
zero/non-zero, and the non-zero value varies by platform. Using _exit()
gives us the full exit code range of values. With exec(), the parent
can't distinguish between a file transfer that succeeds and one that
calls EXCEPT().