Reduce the number of places that know about the number of IP addresses
required by the system tests, by changing `testsock.pl` to read the
`max` from `ifconfig.sh.in`. This should make the test runner fail
early with a clear message when the interfaces have been set up by an
obsolete script.
Add comments to cross-reference `ifconfig.sh.in`, `testsock.pl`, and
`org.isc.bind.system` to make it easier to remember what needs
updating when an IP address is added.
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.
