resolved: handle IDNA domains

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.
resolved: fix negative caching of IDNA domains
resolved: use CLOCK_BOOTTIME instead of CLOCK_MONOTONIC when aging caches and timeing out transactions

That way the cache doens't get confused when the system is suspended.
update TODO
resolved: don't bother caching negative RRs when the SOA TTL is 0 anyway