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.
Unify the header guard style and replace the inconsistent include guards
with #pragma once.
The #pragma once is widely and very well supported in all compilers that
BIND 9 supports, and #pragma once was already in use in several new or
refactored headers.
Using simpler method will also allow us to automate header guard checks
as this is simpler to programatically check.
For reference, here are the reasons for the change taken from
Wikipedia[1]:
> In the C and C++ programming languages, #pragma once is a non-standard
> but widely supported preprocessor directive designed to cause the
> current source file to be included only once in a single compilation.
>
> Thus, #pragma once serves the same purpose as include guards, but with
> several advantages, including: less code, avoidance of name clashes,
> and sometimes improvement in compilation speed. On the other hand,
> #pragma once is not necessarily available in all compilers and its
> implementation is tricky and might not always be reliable.
1. https://en.wikipedia.org/wiki/Pragma_once
Mostly, several functions that take pointers as arguments, almost
always char * pointers, had those pointers qualified with "const".
Those that returned pointers to previously const-qualified arguments
had their return values qualified as const. Some structure members
were qualified as const to retain that attribute from the variables
from which they were assigned.
The macro DE_CONST is used to deal with a handful of very special
places where something is qualified as const but really needs to have
its const qualifier removed.
rdata.c now defines macros for the prototypes of the basic rdata functions,
and all of the lib/dns/rdata/**/*.c files now use them.
Some minor integer-compatibility issues. (IE, ~0x03 is a signed int,
so assigning it to an unsigned int should use a cast. The type of an
enum member is int, so there are some conversion issues there, too.)
A pointers-to-function should not be cast to a pointer-to-object.
Variables should not be named for C reserved identifiers.
One or two set-but-not-used variables removed.
Minor other ISC style cleanups.
