Reduce iptables rule name

[t123yh]

If I use br-External as ExternalInterface, iptables will report an error that shows the rule name is too long. Maybe we'd better use shorter iptables rule name.

[acv]

What's the maximum length?

[databeille]

In openWRT 14.07, chain name's maximum length is 28 chars (iptables says "under 29 chars").

Seems common to every linux : https://git.netfilter.org/iptables/tree/include/linux/netfilter/x_tables.h#n7

[florida63]

see #50