config: Introduce the concept of an "ungettable" variable.

We had though to make all obsolete and invisible variables
ungettable, so that GETCONF would reject them.  But it turns out
that this isn't the current behavior of GETCONF with those
variables.  So for now, I'm leaving the current behavior unchanged.
(See ticket 31647 for a proposal to change the behavior.)

src/app/config/confparse.c

@@ -534,6 +534,23 @@ config_var_is_settable(const config_var_t *var)
   return struct_var_is_settable(&var->member);
 }
 
+/**
+ * Return true iff the controller is allowed to fetch the value of
+ * <b>var</b>.
+ **/
+static bool
+config_var_is_gettable(const config_var_t *var)
+{
+  /* Arguably, invisible or obsolete options should not be gettable.  However,
+   * they have been gettable for a long time, and making them ungettable could
+   * have compatibility effects.  For now, let's leave them alone.
+   */
+
+  // return (var->flags & (CVFLAG_OBSOLETE|CFGLAGS_INVISIBLE)) == 0;
+  (void)var;
+  return true;
+}
+
 bool
 config_var_is_contained(const config_var_t *var)
 {
@@ -776,6 +793,11 @@ config_get_assigned_option(const config_mgr_t *mgr, const void *options,
     log_warn(LD_CONFIG, "Unknown option '%s'.  Failing.", key);
     return NULL;
   }
+  if (! config_var_is_gettable(var->cvar)) {
+    log_warn(LD_CONFIG, "Option '%s' is obsolete or unfetchable. Failing.",
+             key);
+    return NULL;
+  }
   const void *object = config_mgr_get_obj(mgr, options, var->object_idx);
 
   result = struct_var_kvencode(object, &var->cvar->member);