in multiprocessing/forking.py#129, `os._exit` cause child process don't close open
file. For example:
```
from multiprocessing import Process
def f():
global log # prevent gc close the file
log = open("info.log", "w")
log.write("***hello world***\n")
p = Process(target=f)
p.start()
p.join()
```
and the `info.log` will be empty. why not use sys.exit ?
Thanks