Make all BIND libraries private

BIND currently exports number of libraries, but there's virtually no external projects that we are aware of that would be using those libraries. Keeping the ABI and API stable is big burden, and we are exploring possibility of merging all the libraries into a tightly-coupled private library that wouldn't be used outside of BIND (and tools) effectively making those libraries private.

The BIND 9.13/9.14 would be the first release that would drop the libraries.

The BIND 9.11 ESV would keep those libraries until 2022, so any external users would have enough time to migrate to other DNS libraries.

Known external users of libisc and friends:

ISC DHCP (will continue using BIND 9.11 libraries)
dnsperf (either use BIND 9.11 libraries, or make it ISC project)

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