Add system(NULL) to fileio

[Nathan Sidwell]

Corinna Vinschen wrote:

> 
> Wouldn't it be better to add a short comment here explaining the !length
> case, something along the lines of "A zero length indicates a NULL
> argument to system(3) ... handled according to POSIX (check if shell
> exists) even if the system call hasn't been allowed by the user"?

I'll add such a comment, thanks for the review.

nathan

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