avahi: Add systemd init script

This consists of two files:
  - avahi-daemon.service:
    It start/stop the daemon avahi, it's a copy of the S50avahi-daemon
    script.
  - avahi_tmpfiles.conf:
    This one gets installed in /usr/lib/tmpfiles.d/ to be used by
    systemd-tmpfiles which will create the correct directory structure
    with the appropriate rights.
    This is the equivalent of the S05avahi-setup.sh script.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>