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.

It indeed is a macro on Windows.

Make sure we're using the same EventLoop as the NfsManager.

Eliminate dependency on io_thread_get().

More robust and cancellable.