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.
LMDB build variables are already substituted by AC_SUBST() calls in
configure.ac and therefore the latter should not be duplicated in the
AX_LIB_LMDB() helper macro.
The AX_LIB_LMDB() macro attempts to test the potential LMDB installation
path provided to it by temporarily updating CFLAGS and LIBS, calling
AC_SEARCH_LIBS(), and then restoring CFLAGS and LIBS to their original
values. However, including certain statements (e.g. "break") in the
arguments provided to the AX_LIB_LMDB() macro may cause an early exit
from it, in which case CFLAGS and LIBS will be left polluted. Fix by
resetting CFLAGS and LIBS to their original values before executing the
commands provided as AX_LIB_LMDB() arguments.
