Fix XSS in adm_config_report.php

Yelin and Zhangdongsheng from VenusTech http://www.venustech.com.cn/
reported a vulnerability in the Configuration Report page, allowing an
attacker to inject arbitrary code through a crafted 'config_option'
parameter.

Sanitize the parameter prior to output, to ensure HTML special
characters are properly escaped.

Ported from 1.3.x commit c9e5b1d0404503022605459552faeaf610bf15ae.

Fixes 0022579