diagnostics: revamp the handling of -Werror

Recent discussions with Joel E. Denny
(http://lists.gnu.org/archive/html/bison-patches/2013-02/msg00026.html)
show that it is desirable to tell the difference between an option
that was explicitly disabled with -Wno-foo, as opposed to be left
unset.  The current framework does not allow this.

Instead of having a first int to store which options are enabled, and
another to store which are turned into errors, use an array that for
each warning category tells its status: disabled, unset, warning,
error.

* src/complain.h, src/complain.c (warning_bit): New enum.
(warnings): Use it.
(severity): New enum.
(warnings_flag): Now an array of severity.
(errors_flag): Remove, now done by warnings_flag.
(complain_init): New function, to initialie warnings_flag.
(warnings_are_errors): New Boolean, for -Werror.
* src/complain.c (warning_severity): New.
(warnings_print_categories, complains): Use it.
* src/getargs.c (warning_argmatch): Adjust to use warnings_flag.
(warnings_argmatch): Ditto.
Handle -Werror and -Wno-error here.
(getargs): Adjust.
* src/main.c (main): Call complain_init.
* tests/input.at (Invalid options): Add more corner cases.

src/complain.h

@@ -28,17 +28,32 @@
 | --warnings.  |
 `-------------*/
 
+/** The bits assigned to each warning type.  */
 typedef enum
   {
-    Wnone             = 0,       /**< Issue no warnings.  */
-    Wmidrule_values   = 1 << 0,  /**< Unset or unused midrule values.  */
-    Wyacc             = 1 << 1,  /**< POSIXME.  */
-    Wconflicts_sr     = 1 << 2,  /**< S/R conflicts.  */
-    Wconflicts_rr     = 1 << 3,  /**< R/R conflicts.  */
-    Wdeprecated       = 1 << 4,  /**< Obsolete constructs.  */
-    Wprecedence       = 1 << 5,  /**< Useless precedence and associativity.  */
+    warning_midrule_values, /**< Unset or unused midrule values.  */
+    warning_yacc,           /**< POSIXME.  */
+    warning_conflicts_sr,   /**< S/R conflicts.  */
+    warning_conflicts_rr,   /**< R/R conflicts.  */
+    warning_deprecated,     /**< Obsolete constructs.  */
+    warning_precedence,     /**< Useless precedence and associativity.  */
+    warning_other,          /**< All other warnings.  */
 
-    Wother            = 1 << 6,  /**< All other warnings.  */
+    warnings_size           /**< The number of warnings.  Must be last.  */
+  } warning_bit;
+
+typedef enum
+  {
+    /**< Issue no warnings.  */
+    Wnone             = 0,
+
+    Wmidrule_values   = 1 << warning_midrule_values,
+    Wyacc             = 1 << warning_yacc,
+    Wconflicts_sr     = 1 << warning_conflicts_sr,
+    Wconflicts_rr     = 1 << warning_conflicts_rr,
+    Wdeprecated       = 1 << warning_deprecated,
+    Wprecedence       = 1 << warning_precedence,
+    Wother            = 1 << warning_other,
 
     Werror            = 1 << 10, /** This bit is no longer used. */
 
@@ -51,11 +66,25 @@ typedef enum
     Wall              = ~complaint & ~fatal & ~silent
   } warnings;
 
-/** What warnings are issued.  */
-extern warnings warnings_flag;
 
-/** What warnings are made errors.  */
-extern warnings errors_flag;
+/** For each warning type, its severity.  */
+typedef enum
+  {
+    severity_disabled = 0,
+    severity_unset = 1,
+    severity_warning = 2,
+    severity_error = 3,
+    severity_fatal = 4
+  } severity;
+
+/** Whether -Werror was set. */
+extern bool warnings_are_errors;
+
+/** For each warning type, its severity.  */
+extern severity warnings_flag[];
+
+/** Initialize this module.  */
+void complain_init (void);
 
 /** Make a complaint, with maybe a location.  */
 void complain (location const *loc, warnings flags, char const *message, ...)