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.
Update the autosign system test with new expected behavior.
The 'nozsk.example' zone should have its expired zone signatures
deleted and replaced with signatures generated with the KSK.
The 'inaczsk.example' zone should have its expired zone signatures
deleted and replaced with signatures generated with the KSK.
In both scenarios, signatures are deleted, not retained, so the
"retaining signatures" warning should not be logged.
Furthermore, thsi commit fixex a test bug where the 'awk' command
always returned 0.
Finally, this commit adds a test case for an offline KSK, for the zone
'noksk.example'. In this case the expired signatures should be retained
(despite the zone being bogus, but resigning the DNSKEY RRset with the
ZSK won't help here).
