Use #pragma once as header guards

Use #pragma once as header guards

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