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.

Migrate callers to GetSocket().Read(), GetSocket.Write(), which is the
same.

The DynamicFifoBuffer methods never return nullptr when the buffer is
empty or full; instead, they return an empty buffer.  This bug caused
an endless loop.

Switch to the C++ version.

We have a better solution now.

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.

.. and rename it to Flush().

Replaces GLib's GError.

BufferedSocket has just an input buffer, and FullyBufferedSocket adds
the output buffer.