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.

this greatly improves performance of commands that return a lot
of data, e.g. search results or recursive content of a directory,
while being connected to local mpd via tcp/ip socket.

The soure file client.c has nearly 1000 lines, time for splitting it
into smaller pieces to improve readability.

Prepare splitting client.c into several sources.