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().

Allocate a player_control object where needed, and pass it around.
Each "client" object is associated with a "player_control" instance.

This prepares multi-player support.