Rewrite man5/config.pod and related conf code cleanup

Rewrite man5/config.pod and related conf code cleanup

The manpage is basically rewritten.  Use consistent name/value
terminology. Use consistent phraseology to refer to section pointers
and lists of section pointers. Add more cross-references.

Also found a bunch of trivial style things in conf_api.c while
investigating how config works.

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/11431)