Add a generic 'ring the bell' function (console bell on Linux and BSD,
/dev/audio on Solaris), and add DDX functions for this. Make this the
core keyboard's bell.
Port Xvfb and Xnest to this.
Port XFree86 to this, with OS-specific hooks for Linux, BSD, and Solaris
taken from foo_io.c in the old layer.
Daniel Stone