Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

No GLib memory allocation.

Hard-code the program name to "mpd" instead of using g_get_prgname().
The latter has become useless since this GLib variable doesn't get
initialized anymore.

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

Replaces GLib's GError.

More preparations for multi-player support.

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".

Make valgrind happy.

Move code from client_new() and client_close().