Rename includeconf variables for clarity
Rename includeconf variables for clarity

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

Former-commit-id: 33f7a4b53bb25fb0779ed41876cde76041062f33
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin