"LOG_H" is a macro which is also used by ffmpeg/log.h.  This is
ffmpeg's fault, because short macros should be reserved for
applications, but since it's always a good idea to choose prefixed
macro names, even for applications, we are going to do that in MPD.

linux/list.h is a nice doubly linked list library - it is lightweight
and powerful at the same time.  It will be useful later, when we begin
to allocate client structures dynamically.  Import it, and strip out
all the stuff which we are not going to use.