offer guideance on how to make DNS work with firewalld

[rade]

typical firewalld installations have rules that prevent weaveDNS from working. As of #1349 we detect this and show a warning. But we provide no guidance on how resolve the problem. Hence we regularly get queries from users - several in #1266 and also #2192 - and folks end up either disabling firewalld or removing the reject rule that breaks weaveDNS, making the firewall rather less effective in the process.

AFAICT, it should be possible to come up with some firewall-cmds that make weaveDNS work w/o disabling other rules.

[owlab-exp]

This worked for me.

firewall-cmd --direct --add-rule ipv4 filter INPUT 0 -p tcp --dport 53 -j ACCEPT
firewall-cmd --direct --add-rule ipv4 filter INPUT 0 -p udp --dport 53 -j ACCEPT
firewall-cmd --direct --add-rule ipv4 filter FORWARD 0 -p tcp --dport 53 -j ACCEPT
firewall-cmd --direct --add-rule ipv4 filter FORWARD 0 -p udp --dport 53 -j ACCEPT