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.

src/getargs.c

@@ -204,18 +204,18 @@ static const char * const warnings_args[] =
   /* In a series of synonyms, present the most meaningful first, so
      that argmatch_valid be more readable.  */
   "none       - no warnings",
-  "error      - warnings are errors",
   "yacc       - incompatibilities with POSIX YACC",
   "all        - all of the above",
+  "error      - warnings are errors",
   0
 };
 
 static const int warnings_types[] =
 {
   warnings_none,
-  warnings_error,
   warnings_yacc,
-  warnings_all
+  warnings_all,
+  warnings_error
 };
 
 ARGMATCH_VERIFY (warnings_args, warnings_types);
@@ -472,7 +472,10 @@ getargs (int argc, char *argv[])
 	break;
 
       case 'W':
-	FLAGS_ARGMATCH (warnings, optarg);
+	if (optarg)
+	  FLAGS_ARGMATCH (warnings, optarg);
+	else
+	  warnings_flag |= warnings_all;
 	break;
 
       case LOCATIONS_OPTION: