Fix 0012638: allow_url_fopen php.ini setting is boolean

Function url_get( $p_url ) in url_api.php file doesn't work properly
when 'allow_url_fopen' is set to 'Off'. Instead of using a generic
ini_get() call we should instead use the ini_get_boolean() call to get
this PHP configuration value.

Thanks to loginus for reporting this issue and providing this patch.