Pulseaudio alsa configure hook

Pulseaudio alsa configure hook

The attached patch extends the alsa pulse plugin set with a alsa
configuration hook. Allowing one to specify some configuration parameters
that only come into effect when pulseaudio is running.

For example a configution file like:

    @hooks [ {
       func on_pulse_is_running
         pcm.!default { type pulse }
         ctl.!default { type pulse }
       }
   ]

will redirect the default alsa pcm and ctl to pulse iff pulse is running.
(Assuming you defined the hook function correctly ofcourse)

This is usefull for distributions that don't want to force their users to
switch completely to pulseaudio, but have things a bit more dynamic :)

The solutions isn't optimal though. It will mean that every program loading
accessing alsa will try to make an (extra) connection to pulse to decide what
to do. But i think it's the best we can do for now (or at least that i can do
with my minimal knowledge of alsa).

A nicer solution would be a way to always specify the pulse plugin as default
and have a sort of fallback for when that fails.

Signed-off-by: Takashi Iwai <tiwai@suse.de>