MDL-71693 admin: set default for debugdisplay to off.

The previous default value for this config was based on the
PHP ini file `display_error` property (6349a3b). However we
override this propertly during page setup (2533830) according
to the value of the configuration itself.

This had the effect of always setting the default value for
this config to it's current value.

admin/settings/development.php

@@ -38,7 +38,8 @@
     // "debugging" settingpage
     $temp = new admin_settingpage('debugging', new lang_string('debugging', 'admin'));
     $temp->add(new admin_setting_special_debug());
-    $temp->add(new admin_setting_configcheckbox('debugdisplay', new lang_string('debugdisplay', 'admin'), new lang_string('configdebugdisplay', 'admin'), ini_get_bool('display_errors')));
+    $temp->add(new admin_setting_configcheckbox('debugdisplay', new lang_string('debugdisplay', 'admin'),
+        new lang_string('configdebugdisplay', 'admin'), 0));
     $temp->add(new admin_setting_configcheckbox('perfdebug', new lang_string('perfdebug', 'admin'), new lang_string('configperfdebug', 'admin'), '7', '15', '7'));
     $temp->add(new admin_setting_configcheckbox('debugstringids', new lang_string('debugstringids', 'admin'), new lang_string('debugstringids_desc', 'admin'), 0));
     $temp->add(new admin_setting_configselect('debugsqltrace',