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.
Postpone the write using IdleMonitor instead of scheduling a write
event. This reduces the number of system calls, because we don't need
to register and unregister the write event in epoll.
Max Kellermann