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.
some versions of perl failed to run packet.pl because the 'last'
keyword can't be used outside of a loop block. this commit changes
the packet dumping code to a function so we can use 'return' instead.
the tcp system test uses the 'packet.pl' test tool to send a packet
thousands of times. this took a long time because the tool was waiting
for replies and parsing them; however, for that particular test the
replies aren't relevant.
this commit uses non-blocking sockets and moves the reply parsing
outside the send loop, which speeds the system test up substantially.
For some tests, we need to send big data streams (for TCP) or repeated
packets (for UDP), this commits adds `-r` option to packet.pl that sends
the same input <repeats> times using the specified protocol.
