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.
Add tests to the nsupdate system test to make sure that CDS and/or
CDNSKEY that match an algorithm in the DNSKEY RRset are allowed. Also
add tests that updates are rejected if the algorithm does not match.
Remove the now redundant test cases from the dnssec system test.
Update the checkzone system test: Change the algorithm of the CDS and
CDNSKEY records so that the zone is still rejected.
