Werk #7177: Fix background job warning "IOError: [Errno 9] Bad file descriptor"

When starting a background job an unproblematic error message like this could occur in the output of the background job log:

sys.stdin.close() IOError: [Errno 9] Bad file descriptor

This was caused by an unclean stdin/stdout/stderr handling during job startup since commit b15dfeb619c14c925b66f2a3437d213852204975.

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