includeconf -> conf_file_names
to_include -> conf_file_name
include_config -> conf_file_stream
Suggestion from John Newbery <john@johnnewbery.com> in
https://github.com/bitcoin/bitcoin/pull/15934#discussion_r343905138
This is a partial backport of Core PR15934 : https://github.com/bitcoin/bitcoin/pull/15934/commits/5a84aa880f6da0bac0e2144733fdef3b8558c761