ap-mdns: restart avahi-daemon when a new network is added or removed

There is a 10 second delay to avoid problems caused by too fast
avahi-daemon spawning/killing. Without delay, avahi-daemon seems to
produce zombies.
ap-mdns: rename function
ap-mdns: remove created service files afterwards
ap-mdns: simplify iface flag getter
ap-mdns: do not choke on avahi-daemon -k
ap: maintain network list in run dir
ap-mdns: restart avahi-daemon only if there's changes in networks
ap-mdns: remove previously written avahi services before avahi restart

This ensures that the list of services stays up to date through out the
lifetime of puavo-wlanap-mdns.
ap: read PWAP_RUNDIR from configs
ap-mdns: skip empty sets on mdns filter handling
ap-mdns: refresh mDNS filters only if puavo-wlanap's networks file exists

It does not exist when puavo-wlanap is not running (might be restarting
or not event started yet).
ap-mdns: add few simple trace outputs
ap: simplify networks file management
ap: support multiple SSIDs in legacy mode

They are all just bound to the same bridge.
ap: hostapd support max 4 SSIDs per radio
ap: support AP-specific SSIDs

This lets us configure AP-specific networks easily via Puavo. The Magic
SSID is chosen to be longer than 32 characters, since 32 is the maximum
length of a valid SSID, as defined in IEEE 802.11
(http://standards.ieee.org/getieee802/download/802.11-2007.pdf). Moreover,
since puavo-wlanap uses SSIDs as interface names, they must be shorter
than 16 characters.