Cleanup libirs APIs and slim down the library to just irs_resconf
The libirs contained own re-implementations of the getaddrinfo, getnameinfo and gai_strerror + irs_context and irs_dnsconf API that was unused anywhere in the BIND 9.
Keep just the irs_resonf API that is being extensively used to parse /etc/resolv.conf by several of BIND 9 tools.
Closes #1758 (closed)