errors: change output, and improve -y coherence

errors: change output, and improve -y coherence

The prefix of warnings treated as errors is now "error: ". Also, their
suffix now reflects the changes in the Werror option format.

An output for -Werror=other used to be:
  bison: warnings being treated as errors
  input.y:1.1: warning: stray ',' treated as white space [-Wother]

It is now:
  bison: warnings being treated as errors
  input.y:1.1: error: stray ',' treated as white space [-Werror=other]

The line "warnings being treated as errors" no longer adds any info,
it will be removed in a forthcoming change.

* NEWS: Add entry "Enhancement of the -Werror"
* doc/bison.texi: Move the warnings-as-error to a new bullet.
* src/complain.c (complains): Refactor, change the prefix of warnings
that are treated as errors.
(warnings_print_categories): Support for [-Werror=CATEGORY] display
* src/getargc.c (getargs): -y implies -Werror=yacc
* tests/input.at: Update expected --yacc output for coherence.

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