config: make changes to allow some parts to build as C++

[kevans]

Summary

Highlights:

• Avoid keywords (this, not) as variable names
• Move yyparse into config.h with other declarations
• All declarations in config.h are assumed guilty until proven innocent
• Some const-correctness
• Casting malloc/calloc returns

Note that we're not building any C++ here yet, this will be introduced
in other commits to replace some of the lib dependencies.

Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.

[imp]

This looks good and is quite mechanical.
My only question is implicit reliance on sys/cdefs.h being included... but that's going to give a compile error if that becomes not the case in the future.

usr.sbin/config/config.h

45

Do we know for sure sys/cdefs.h has been included?

usr.sbin/config/main.c

701

It's unfortunate we have to do this :(

[kevans]

usr.sbin/config/config.h

45

We do in the sense that sys/types.h unconditionally includes it, but I' can go ahead and make it explicit