proxy.cgi: Further fix for bug 13893

proxy.cgi: Further fix for bug 13893

- Previous patch for proxy.cgi was related to the mitigation provided by the bug reporter
   for the parameter VISIBLE_HOSTNAME. This parameter however was not mentioned in the
   description for that bug.
- bug 13893 description mentions TLS_HOSTNAME, UPSTREAM_USER, UPSTREAM_PASSWORD,
   ADMIN_MAIL_ADDRESS, and ADMIN_PASSWORD but it mentions them as being from dns.cgi
   which is incorrect except for TLS_HOSTNAME.
- The other parameters are from proxy.cgi but no mitigation was shown for those in the
   bug report.
- This patch adds fixes for the parameters UPSTREAM_USER, UPSTREAM_PASSWORD,
   ADMIN_MAIL_ADDRESS, and ADMIN_PASSWORD

Fixes: bug 13893 - proxy.cgi Multiple Parameters Stored Cross-Site Scripting
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>