Let's turn resolved into a something truly useful: a fully asynchronous DNS stub resolver that subscribes to network changes. (More to come: caching, LLMNR, mDNS/DNS-SD, DNSSEC, IDN, NSS module)
These are enough calls for a new file, and they are sufficiently different from the sockaddr-related calls, hence let's split this out.
Also remove the equivalent functionality from networkd.