warnings: display warnings categories

warnings: display warnings categories

* src/complain.c (error_message): Call 'warnings_print_categories'.
* src/gram.c (grammar_rules_useless_report): Display itself warning
category.
* tests/actions.at, tests/conflicts.at, tests/existing.at,
tests/input.at, tests/named-refs.at, tests/output.at, tests/reduce.at,
tests/regression.at, tests/skeletons.at: Adjust.
* NEWS: Document this.

Signed-off-by: Akim Demaille <akim@lrde.epita.fr>