Finish implementing --warnings=error, which should not be implied by

Finish implementing --warnings=error, which should not be implied by
--warnings=all (or by its synonyms -W and --warnings without
subarguments).
* src/complain.c (set_warning_issued): New function to report that
warnings are being treated as errors and to record an error if so.
Invoke...
(warn_at, warn): ... here.
* src/getargs.c (warnings_args, warnings_types): Reorder so that
"error - warnings are errors" does not appear above "all - all of the
above".
(getargs): For -W and --warnings without subarguments, don't let
FLAGS_ARGMATCH set warnings_error in warnings_flag.
* src/getargs.h (enum warnings): Unset warnings_error in warnings_all.