Complain about unused %define variables and %code qualifiers.

Complain about unused %define variables and %code qualifiers.

* NEWS (2.5): Document.
* data/bison.m4 (b4_check_user_names): Complain instead of warn.
* doc/bison.texinfo (Decl Summary): Document complaint, and
improve %define documentation a little otherwise.
* tests/input.at (Reject unused %code qualifiers): Update.
(%define errors): Update.
(%define, --define, --force-define): Update.
(%define backward compatibility): Update.
(Unused %define api.pure): Update.
* tests/push.at (Push Parsing: Unsupported Skeletons): Update.