Add support for IPv6

[GregoryHinton]

Create IPv6 firewall rules that are as close as possible to the IPv4 rules, by adding writes to the /proc/sys/net/ipv6 subtree and invocations of the /sbin/ip6tables command. Both of these are almost identical to their IPv4 counterparts; but, unfortunately, just different enough to require some "uglification" of the shell script. I tried to balance ease of maintenance in future revisions with readability. Hope you find these changes useful.