osx: add native zeroconf implementation via Bonjour

osx: add native zeroconf implementation via Bonjour

Avahi and dbus is too heavy for OSX just for the sake of publishing our
services via mDNS/Zeroconf. Apple has its own Zeroconf implementation
called Bonjour, and this patch adds a module that implements service
announcement with that API.

All data gathering is copied from module-zeroconf-publish.c, but
unfortunately the code there is too specifically made for avahi, so I
couldn't factor it out to reuse it.