Rename includeconf variables for clarity

Summary:
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

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5885