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.
The test spawns 4 parallel workers that keep adding, modifying and
deleting zones, the main thread repeatedly checks wheter rndc
status responds within a reasonable period.
While environment and timing issues may affect the test, in most
test cases the deadlock that was taking place before the fix used to
trigger in less than 7 seconds in a machine with at least 2 cores.
