Support advanced DNS queries

[chrysn]

In light of work on transport-indication, it would be good to be able to get more DNS results than just A / AAAA records back from a query: SVCB requests would be interesting, as would be TLSA records for setting up TLS.

The current getaddrinfo based interface doesn't provide anything exceeding A/AAAA records.

Trouble is: getaddrinfo appears to be the last portable thing.

• On the C side there is libresolv. No clue how well that works outside of Linux. On Android (termux) it works with the libresolv-wrapper package, provided its libresolv_wrapper.so is symlinked to libresolv.so and LD_PRELOAD=. (or comparable tricks). A Python ctypes wrapper exists and would need some patching to do anything but MX.
• There are various DNS based tools that'll ask the system for its resolver, eg. dnspython or Rust's hickory_resolver. They don't really do anything other than read /etc/resolv.conf.
• Termux has a whole discussion on Replace resolv.conf usage with android default nameserver(s)? termux/termux-packages#1174. They've contemplated running a local resolver that'll convert into the Android internal format, but that'd be rather tedious, and anyway isn't happening. (Seems systemd does something similar).