Clean shutdown of the DNS-SD registration on ippusbxd shutdown
Clean shutdown of the DNS-SD registration on ippusbxd shutdown

Following the Avahi documentation we lock the Avahi poll thread now
when removing the printer registration in the dnssd_shutdown()
function and we also explicitly stop this thread now. This avoids
segmentation faults during the shutdown of the daemon.

daniel-dressler/ippusbxd,tillkamppeter/ippusbxd