Check if other firewall's are on the system on start

[Jamie Strandboge]

ufw's check-requirements added a check to see if other firewalls are on the system since that can interfere with ufw's ability to operate. It might be good to do this on start in some tasteful way. That said, we would want to avoid false positives since just because a firewall is on the system, doesn't mean it is configured for use (though, some firewalls default to 'on', like iptables-persistent).