Fix Windows script process handle leaks

[wolph]

Description

I noticed that nzbget was leaking more and more over time, so I had codex investigate and write up a fix.

I manually reviewed all of the code changes that codex made and they look good to me, but I'm not that familiar with the nzbget codebase yet so please let me know if I missed something.

AI assistance

• This PR involved AI assistance (see AI Policy)

Lib changes

Testing

I've tested it for quite a while and I haven't had any leak issues since.

[wolph]

If you want the fix to go in a different direction let me know too. I was pondering if it would be better to have the #ifdef WIN32 bit inside of the SetProcess so we can just call it from every bit of code and it would be a no-op for other platforms. The compiler should collapse it anyhow.

[dnzbk]

@wolph The handle leak fix looks correct. One suggestion: use std::mutex + std::lock_guard instead of the custom Mutex/Guard - the custom ones are legacy.

[wolph]

@wolph The handle leak fix looks correct. One suggestion: use std::mutex + std::lock_guard instead of the custom Mutex/Guard - the custom ones are legacy.

Done :)

My C++ is a bit rusty ;)