Signed-off-by: Ted Hess <thess@kitschensync.net>

As discussed in #543 provide the avahi service file within its own
tiny package to allow the user to choose which services are
announced.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>

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The former avahi service file was only installed for mpd-full. Does
the mpd-mini does not support network functionality?

Then we should adjust deps here...

Signed-off-by: Ted Hess <thess@kitschensync.net>