std.file functions and embedded NUL characters [CWE-158]

[kdevel]

On Thursday, 31 July 2025 at 22:38:41 UTC, kdevel wrote:
> Unfortunately isValidPath also flags perfectly legal filenames
> as invalid, e.g. such containing invalid UTF-8 sequences.

I stand corrected. This is probably not true:

ivp.d:
```
    import std;

    void main (string [] args)
    {
       auto input = "a\xfc";
       writeln (input, ": ", isValidPath (input));
    }
```

$ ./ivp [latin9 mode console]
aü: true