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.