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

[Richard (Rikki) Andrew Cattermole]

This is quite a good example of why for PhobosV3 I want us to go through 
a FilePath abstraction, rather than accepting random strings for file names.

This is indeed a security vulnerability, but it isn't on D's side.
All system API's take in a null terminated string, when it should've 
been pointer + length.

If someone has a problem with this currently, you can call 
``isValidPath`` in ``std.path``, which will check for the null character.

https://dlang.org/phobos/std_path.html#isValidPath