add alternate path for iptables binaries on archlinux

The iptables binaries on archlinux are under /usr/sbin/ rather than /sbin/
Symlinking the binaries was my initial workaround, but I don't intend to do that on all my machines.

I put in a simple case statement and it seems to work.
Note that I'm not a programmer and have no clue about ruby, so I'm guessing there might be better/cleaner ways to implement this?

Also the firewallchain stuff will probably have to be adapted in the same way, I'll take a look at that after I get feedback on how to implement this.

Signed-off-by: Dave Simons <dave@inuits.eu>