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.
This mostly removes stuff that's either deprecated, obsolete or not used
at all:
* Update the minimal autoconf version to 2.69
* AC_PROG_CC_C99 is deprecated, just use AC_PROG_CC as we require C11
anyway
* AC_HEADER_TIME is deprecated, both <sys/time.h> and <time.h> can be
included at the same time, and we don't use the macros that
AC_HEADER_TIME defines anywhere
* AC_HEADER_STDC checks for ISO C90 and we require at least C11
* Replace AC_TRY_*([]) with AC_*_IFELSE([AC_LANG_PROGRAM()])
* Update m4/ax_check_openssl.m4 from serial 10 to serial 11
* Update m4/ax_gcc_func_attribute.m4 from serial 10 to serial 13
* Update m4/ax_pthread.m4 from serial 24 to serial 30
* Add early AC_CANONICAL_TARGET call to prevent warning from AX_PTHREAD
