fix WebUI system information leak

fix WebUI system information leak

Disable unauthenticated access to cgi-bin/credits.cgi. The page
leaks the currently installed version of IPFire and the hardware
architecture.

Both information might make a successful attack much easier.

This issue can be reproduced by accessing https://[IPFire-IP]:444/cgi-bin/credits.cgi
and accepting a SSL certificate warning (if any).

Signed-off-by: Peter Müller <peter.mueller@link38.eu>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>