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.
In the "allow-query" system test, ns3 uses a root hints file which
contains a single entry for a.root-servers.nil (10.53.0.1). This name
is not present in the root zone served by ns1, which means querying it
for that name and any type will yield an NXDOMAIN response. When
combined with unfavorable thread scheduling, this can lead to ns3
caching an NXDOMAIN response for the only root server it is aware of and
thus to false positives for the "allow-query" system test caused by ns3
returning unexpected SERVFAIL responses. Fix by modifying the root zone
served by ns1 so that authoritative responses to a.root-servers.nil
queries match the root hints file used by ns3.
