CUPS mDNS SRV RRs (was: Better control over what mDNS announces)

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

> Maybe stupid question: wouldn't CUPS cooperate with the static DNS
> method to enumerate services for Bonjour?
Probably a mixture works. The only problem with the mDNS RRs that CUPS emits 
is that the server name referenced to in the SRV record is foo.local, wich 
then needs to be resolved by mDNS, an that's where I'm losing control.

So I added an option to CUPS to tell it what to put in the SRV RR instead. 
That way, I can make it emit cups.math.uni-bonn.de, which resolves via 
unicast DNS and I'm back in control.

See https://github.com/apple/cups/issues/5071.

(I also implemented mDNSResolver options to
-- only hand out the first non-LL address of an interface for each family
-- don't hand out LL addresses
-- ignore an interface/family combination with no routable address)