stage_file_native.cpp issues

[davidpanderson]

Problems with this:

1. It generates compile warnings (unused vars, shadowed vars)
2. It has no comments or documentation
3. the PR says it's a library function, but it has main() so it can't
   be used as a library function

Should this be removed?
It's redundant with the PHP script stage_file

[AenBleidd]

This was added here: #4733

I believe, it's still useful to have it, so maybe we should just fix the issues and the documentation?

[davidpanderson]

It's C++ code that does the same thing as existing PHP code.
If it's significantly faster maybe it's useful,
but it's not clear that it is faster.

[AenBleidd]

I remember there was a reason for that. Just don't recall which exactly.
I'll try to find out and we can consider how to proceed with that later.