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.
All signed zone files present in bin/tests/system/inline/ns8 should
contain the unsigned serial number in the raw-format header. Add a
check to ensure that is the case. Extend the dnssec-signzone command
line in ns8/sign.sh with the -L option to allow the zones initially
signed there to pass the newly added check. Add another zone to the
configuration for the ns8 named instance to ensure the check also passes
when multiple zones are inline-signed by a single named instance.
