Allow declaring a parameter unused when not DEBUGGING

[khwilliamson]

This is a follow on to b35a6ea which adds the ability to decare a parameter to be unused. This new commit makes it convenient to declare a parameter to be DEBUG_ONLY, allowing the removal of PERL_UNUSED_ARG statements in the code that the previous commit didn't.

And this commit removes two related such statements, to show how it works.

The purpose of PERL_UNUSED_ARG is to avoid expected warnings that are known harmless, without turning off all warnings of that kind. It is possible to use that macro when there are actual uses of the parameter, thus defeating the macro's purpose to some extent. This is typically done when some Configure options cause the parameter to be used, and some do not, and the author didn't want to add the extra #ifdef's required to get the macro to be in effect in just the precise conditions. After all, there's added complexity in the code for both the author and future readers that can obscure the basic logic, with not that much gain.

But, the most common case where the parameter is only used in some Configurations is in DEBUGGING builds. This commit handles this common case. It is a simple extension to
b35a6ea, to whose commit message you are referred to the details.

This is enabled only for core perl and extensions.

To use this capability, the symbol DEBUG_ONLY is added after the parameter name in both its embed.fnc entry and its definition.

• This set of changes does not require a perldelta entry.