Normalize include guards to FS_<FILENAME>_H

[ranisalt]

Pull Request Prelude

• I have followed proper The Forgotten Server code styling.
• I have read and understood the contribution guidelines before making this PR.
• I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

Let's have some standardization here. Prepended FS_ to every guard to avoid name clashes on common names.

We could alternatively use #pragma once instead, the code style suggests it but it was never really used.

[Erza]

I like the pragma once idea, it's a compiler extension, but basically every single compiler supports it, and it's cleaner than these custom include guards.

https://en.wikipedia.org/wiki/Pragma_once#Portability

[ranisalt]

Just to summarize:

Include guards

• Defined in the standard 👍
• Optimized by compilers 👍
• Works with symlinks (that we should not use anyway) 🤷
• Allows importing the same file from multiple places (useful for libraries, not for us) 🤷
• Error prone (mismatched #ifndef/#define tokens, missing #endif) 👎
• Prone to collision (i.e. two files using the same token) 👎

#pragma once

• Optimized by definition 👍
• Two lines less 🤷
• Does not work with symlinks 🤷
• Never collides, but allows importing twice from different places 🤷
• Not standard (but available in every compiler that makes sense) 👎