modified detection

• 500862tenable_ot_schneider_CVE-2017-6026.nasl • 1.3
• 500870tenable_ot_schneider_CVE-2018-7789.nasl • 1.4
• 500858tenable_ot_schneider_CVE-2018-7790.nasl • 1.4
• 500864tenable_ot_schneider_CVE-2020-7566.nasl • 1.3
• 500851tenable_ot_schneider_CVE-2020-7565.nasl • 1.3
• 500298tenable_ot_schneider_CVE-2017-7574.nasl • 1.6
• 500871tenable_ot_schneider_CVE-2017-6028.nasl • 1.4