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.

We can do CURL requests asynchronously, and we don't need a
synchronous WorkQueue thread for that.

This allows parallelizing lookups and allows immediate cancellation.

The old struct config_param remains only for top-level string options.