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.
irs_resconf_load() stores the value returned by add_search() into ret
without consulting its current value first. This causes any previous
errors raised while parsing resolv.conf to be ignored as long as any
"domain" or "search" statement is present in the file.
Prevent this by returning early in case an error is detected while
parsing resolv.conf. Ensure that "searchlist" and "magic" members of
the created irs_resconf_t structure are always initialized before
isc_resconf_destroy() is called.
