Audio forwarder.

Copyright 1999 Pete Wyckoff <wyckoff@ca.sandia.gov>.

This is a pre-release, and might oops your machine.  It seems stable
enough on mine right now, regardless of which daemons are up/down, and
where Ctrl-C lands in various processes, but I may have missed some corner
cases.

Comments are welcome, in particular on nice ways to expand this to
handle arbitrary character devices, and how not to have to hard-code
all known ioctls into the code.  Patches are even more welcome.

It's only been tested with linux kernels 2.3.3 and 2.3.5, both uniprocessor
and SMP versions.  Glibc version 2.1.

The players:

    client machine - no soundcard

	fakeaudio.o module - listens on char device major 14,
	    and responds for /dev/audio, /dev/dsp, /dev/mixer.
	    Forwards system calls to the userspace daemon.
	
	afwd client userspace program - opens /dev/audio in a
	    special way to tell the module it's the process that
	    will forward system calls to some other machine.  Opens
	    a socket to the server machine.

	generic audio program - e.g. "cat foo.au > /dev/audio".  Calls
	    open/read/write/ioctl which are handled by the fakeaudio.o
	    module.

    server machine - has soundcard

	afwd server userspace program - start with "afwd -r".  Listens
	    for socket connections, receives requests from client,
	    performs them locally, and returns the results.

Only one audio client at a time is supported, just like on a single machine.
For mixing, check out EsounD (ref?).

To start it all:

    server$ afwd -r &
    client# insmod fakeaudio
    client$ ./afwd &
    client$ cat incoming_call.au > /dev/audio

To shut it down nicely:

    client$ echo kill all audio users
    client$ killall afwd
    client# rmmod fakeaudio
    server$ killall afwd

The afwd daemon on the server must have permissions to open the audio
devices, which are all 666 on my machine.  Also there's no client
authentication, but I only use it behind my firewall.  Again, patches
are welcome!