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.
Each zone used in the "inline" system test contains a few dozen records.
Over a dozen of these zones are used in the test. Most records present
in these zones are not subsequently used in the test itself, but all of
them need to be signed by the named instances launched by the test,
which puts quite a bit of strain on lower-end machines, leading to
intermittent failures of the "inline" system test. Remove all redundant
records from the zones used in the "inline" system test in order to
stabilize it.
