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.
Remove the runtime check for eventfd(), hard-code the feature once
it's been selected at compile time. The class WakeFD is splitted into
EventFD and EventPipe, using WakeFD as a macro diversion.
