Modernize include guards and add missing ones

[Awkor]

This PR converts the remaining #ifndef header guards to #pragma once.

I used guard2once from guardonce to do the conversion.

[ClobberXD]

I'm pretty sure the #ifndef was left untouched on purpose, because there are two header files with the same name:

• src/hud.h
• src/client/hud.h

Would #pragma once take this quirk into account?

[Awkor]

From what I've gathered from here, here and here, I think this quirk should be handled correctly.

[ClobberXD]

That does indeed seem to be the case, thanks for clarifying.

[orbea]

#pragma once is not standardized, ifdef guards are. This seems much more likely to introduce problems than solve them.

[SmallJoker]

@orbea A while back #pragma once was decided to be widely supported, and thus accepted as header guard for Minetest. Somewhere there's a PR about this.

@Awkor While you're at it - would you please be so nice and include the missing guards as mentioned in #9899 (comment) ?

[ClobberXD]

#pragma once is supported on all mainstream compilers, even if it isn't a part of the C++ specification itself. See https://en.wikipedia.org/wiki/Pragma_once#Portability

[Awkor]

@SmallJoker Done.