Make sure we format UTF-8 labels as IDNA when writing them to DNS
packets, and as native UTF-8 when writing them to mDNS or LLMNR packets.
When comparing or processing labels always consider native UTF-8 and
IDNA formats equivalent.
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)