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.

Fixes a crash that can occur due to recursion from InvalidateSockets()
to ReadInfo() to CurlRequest callbacks.

The CurlRequest gives us a more low-level API to CURL without the
InputStream interface, integrated into our IOThread.