[mythtv] No Start with Pulse Audio (Changeset 20310)

[cal]

further to danielk's prior comment, his comment on ticket #5473 could also be
relevant(?):-

> (In [20421]) Refs #5473. Add code to deal with pulse audio.
>
> By default when compiled on a system with pulse audio we just tell the
> pulse audio server to leave us alone. For those that want to experiment
> with pulse audio, we now read the EXPERIMENTALLY_ALLOW_PULSE_AUDIO
> environment variable and when it is set allow MythTV to run with pulse
> audio enabled.