This commit converts the license handling to adhere to the REUSE
specification. It specifically:
1. Adds used licnses to LICENSES/ directory
2. Add "isc" template for adding the copyright boilerplate
3. Changes all source files to include copyright and SPDX license
header, this includes all the C sources, documentation, zone files,
configuration files. There are notes in the doc/dev/copyrights file
on how to add correct headers to the new files.
4. Handle the rest that can't be modified via .reuse/dep5 file. The
binary (or otherwise unmodifiable) files could have license places
next to them in <foo>.license file, but this would lead to cluttered
repository and most of the files handled in the .reuse/dep5 file are
system test files.
3943. [func] SERVFAIL responses can now be cached for a
limited time (configured by "servfail-ttl",
default 10 seconds, limit 30). This can reduce
the frequency of retries when an authoritative
server is known to be failing, e.g., due to
ongoing DNSSEC validation problems. [RT #21347]
