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

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. Reducing the
number of FreeBSD-specific dependencies we have reduces some friction
for building our bootstrap tools independently in other environments.

Reviewed by: imp
Sponsored by: Klara, Inc.
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D38274