Change from audio group to something else

[Kaj Ailomaa]

I just reported a bug on the usage of audio group. bug #766914

As it seems to be considered a bad group for users to belong to, and the
fact that jackd packages expects the users to be apart of it, I propose
to remove audio as a user default and create a new group for jack
specifically.

ffado also requires the audio group, but AFAIK, the ffado drivers are
only used with jack.

Any thoughts on this?