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.
Pull and use several autoconf archive convenience macros to simplify
configure.ac.
* AX_CHECK_COMPILE_FLAG(FLAG, ...) - check whether given CFLAG works
* AX_CHECK_LINK_FLAG(FLAG, ...) - check whether given LDFLAG works
* AX_CHECK_PREPROC_FLAG(FLAG, ...) - check whether give CPPFLAG works
* AX_SAVE_FLAGS/AX_RESTORE_FLAGS - save and restore *FLAGS
