Since we switched from autotools to Meson in commit
94592c1406, we don't need to include
`config.h` early to properly enable large file support. Meson passes
the required macros on the compiler command line instead of defining
them in `config.h`.
This means we can include `config.h` at any time, whenever we want to
check its macros, and there are no ordering constraints.
With this commit, multi-player support becomes possible... it's just
not wired to the frontend yet.
This is based on massive amounts of refactoring work I did over the
past 9 years.
