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.

Prepare to make it mutable.

Referencing the attribute "partition" is illegal after Close(),
because Close() deletes "this".

Replaces global variable "main_loop".

Add a new CommandResult code called "FINISH" which, unlike "CLOSE",
will attempt to flush the output buffer.  This is a one-shot attempt;
it will do one write, and not try again.

Remove the const_cast from HttpdClient.cxx, and avoid one allocation
in ClientRead.cxx.

Don't use a global loop over the whole client list.