resolved: add "proxy-only" stub on 127.0.0.54

resolved: add "proxy-only" stub on 127.0.0.54

This beefs up the DNS stub logic to listen on two IP addresses:
127.0.0.53 (as before) + 127.0.0.54 (new). When the latter is contact
our stub will operate in "bypass" mode only, i.e we'll try to pass DNS
requests as unmodified upstream as we can (and not do mDNS/LLMNR and
such, also no DNSSEC validation – but we'll still do DNS-over-TLS
wrapping).

This is supposed to be useful for container environments or tethering:
this stub could be exposed (via NAT redirect) to clients of this system
and we'll try to stay out of the way with doing too much DNS magic
ourselves, but still expose whatever the current DNS server is from
upstream under a stable address/port.

How to use this:

  # iptables -t nat -I PREROUTING -p udp -i <interface> --dport 53 -j DNAT --to 127.0.0.54:53
  # echo 1 > /proc/sys/net/ipv4/conf/<interface>/route_localnet