[Cleanup] Unify header guards

[Fuzzbawls]

For a while now, we've been using a mish-mash of various header guard styles, with no real consistency project wide. This PR remedies that by setting a strict header guard notation style.

All first-party headers now follow the below guard style:

#ifndef PIVX_<PATH_TO>_<FILENAME>_H
#define PIVX_<PATH_TO>_<FILENAME>_H
...
#endif // PIVX_<PATH_TO>_<FILENAME>_H

Git subtrees and integrated 3rd party libraries are excluded.

A linter is included so as to prevent future inclusion of include guards that do not adhere to the above standard style.

With this in place, there is more opportunity to "declutter" our file structure quite a bit, especially with regards to the GUI files.

[panleone]

only one small nit:

with the current script we check only that the header guard begins with PIVX_<PATH_TO>_<FILENAME>_H

But something like
PIVX_<PATH_TO>_<FILENAME>_H2323asd

would be accepted too.

I suggested a small change to fix the issue