Formalize symbol names reserved for Perl's use in XS

[khwilliamson]

This creates a regular expression pattern of names that we feel free to expose to XS code's namespace. Hence they are names reserved for our use, and should any conflicts arise, the module needs to change, not us.

Naturally, the pattern is pretty restrictive.

Any symbol beginning with "PL_"
Any symbol containing perl, Perl, or PERL, usually delimitted on
    both sides so as to keep it from being part of a larger word.

Any other spelling that we expose could be considered to pollute the XS code space. We have felt free to do that all the time. Any new function's short name will do that.

And we generally feel free to create macros with arbitrary names which could conflict with an existing XS name.

Some important potential conflicts are:

New keywords: We create an exposed KEY_foo macro. Some existing modules use some of these. My grep of CPAN shows maybe a dozen of these get used; mostly KEY_END.

config.h is full of symbols like HAS_foo, I_bar, and others that are all exposed. I don't imagine we can claim to reserve any symbol beginning with either HAS_ or I_. And I don't know what to do here.

Informally, myself and others have used a trailing underscore to indicate a private symbol. There are a few distributions that use some of these anyway. And there has been pushback when new short symbols that use this convention have been added.

I would like to get a formal rule about use of this convention. There are 200+ of these currently. We could reserve any names with trailing underscores, or if that is too much, any ending in, say, 'pl' or 'PL'.

We have 3000+ undocumented macro names that don't end in underscores and which are currently visible to XS code. This number includes the KEY_foo ones, but not the ones in config.h.

To deal with namespace pollution, we have had the -DNO_SHORT_NAMES Configure option for use just with embedded perls. This hasn't worked at least since we added inline functions, and it always applied to only functions. I have a WIP to get this to work again, and to extend it to work with documented macros. It just occurred to me how to make this be customizable, so that downstream someone could add a list of symbols that should only exist as 'Perl_foo', and then recompile, leaving short names for everything not in the list.

• This set of changes requires a perldelta entry, and I need help writing it.