ap-mdns: setup event listener before handling cached events

Previously it was possible that some events got lost after having
handled all cached events but before we had an event listener up and
running.
ap: maintain a list of current networks

This list can be used by other components suchs as ap-dnsproxy or
ap-mdns. Currently they are maintaing their own versions.

Move also the network setup to library scripts to ensure that the
network is added/removed before script parts are being run.
ap-mdns: use the network list mantained by ap
ap-dnsproxy: use the network list maintained by ap
ap-dnsproxy: use port 53533 instead of 5353 which is standard mDNS port
ap-mdns: simplfy the default avahi daemon conf
ap-mdns: simplify iptables management
ap-mdns: refactor iptables management
ap-dnsproxy: simplify iptables management