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.

Publish the Client API, preparing to move more code into the Client
class.

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

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

No caller needs this.  Fixes use-after-free after returning from
Client::OnTimeout().

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

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

playlist and player_control are deprecated.

Reduce access to the global variable "g_playlist".

Random access is not necessary here.

Move code from client_new() and client_close().