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

Replaces GLib's GError.

Eliminate the C wrappers.

Don't use va_list.

Reset the timer when paused and no client is connected.

This fixes Mantis ticket 0003527.

Wrap close(), use closesocket() on WIN32/WinSock.

Pass audio_output objects around instead of void pointers.  This will
give some more control to the plugin, and prepares for non-blocking
audio outputs.

Move the "extern" declarations from output_list.c, for more type
safety.

In the "vorbis" plugin, this is a copy of the old flush() method,
while flush() gets a lot of code remove, it just sets the "flush" flag
and nothing else.  It doesn't start a new stream now, which should fix
a few problems in some players.