Revise default $g_display_errors setting

The default now reflects the recommended Production settings, and a
better recommendation for developers is documented in the phpDoc
comment block.
Isn't it some kind of security issue if this is the default setting for localhost?
Every user working on the same server will use the settings.
It's better to change it only if needed in config_inc.php.

I hope you will revert the behavior, if not you should consider also IPv6.