Adjust cf3/cf.squid

Rather rely on Squid's built-in restart mechanism than on systemd; this
is needed in case of separate partitions for /var and /var/spool/squid;
'ExecReload=/bin/kill -HUP $MAINPID' doesn't work because squid.pid is not
yet available. While 'systemctl reload squid' will fail, it's still
possible to use 'service squid reload', used by 'squid-update-cachedir'.