Handle closed sys.__stdin__, for multiprocessing

Handle closed sys.__stdin__, for multiprocessing

Buggy code in python's multiprocessing/process.py closes sys.stdin and
reassigns it to open(os.devnull), but fails to update the corresponding
__stdin__ reference. So, detect that case and handle it appropriately.
The buggy code is visible in http://hg.python.org/lookup/r73708.