OK, this is the big one. We still keep using `#include` guards only
where we absolutely need to, but with each header now being valid in
isolation, this can now actually help *minimize* the length of each
translation unit's `#include` list. Turns out that after removing all
the duplicates, we only *actually* need to guard 29 headers across all
5 games.
Part of P0285, funded by [Anonymous] and iruleatgames.
The `struct date` members in `scoredat_t` add a dependency on <dos.h>
to the entire header file. Makes sense to move it out as early as
possible.
Part of P0133, funded by [Anonymous].