Since we switched from autotools to Meson in commit
94592c14, 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.

These attributes are printed in the "outputs" response, and the new
command "outputset" allows setting new values.

No attributes are currently implemented.

Wrapper for protocol/ArgParser.cxx.

Another big chunk of code for multi-player support.