proxy.cgi: Re-apply accidentially removed changes.

Removed in dfee7582f9b386126fcaa6c8cdcb98677e34f5b4,
originally introduced in 49c7cb232870cce6986f93bb6fc3b6dbf643ec07.
squidclamav: Fix permissions of /etc/squidclamav.conf.

The file must not be executable, but writeable by anybody
in the group nobody.
squidclamav: Fix indentation of update script.