Favor REDIRECT over DNAT

[willglynn]

The README currently suggests intercepting traffic using -j DNAT --to-destination $IP:$PORT, but there's an alternative:

Redirection

There is a specialized case of Destination NAT called redirection: it is a simple convenience which is exactly equivalent to doing DNAT to the address of the incoming interface.

This commit changes the README to suggest -j REDIRECT --to-ports $PORT instead. This saves the step of needing to find the interface IP (eliminating the ip|grep|awk|cut|head chain) which makes configuration simpler. Additionally, REDIRECT is guaranteed to always send matching traffic to the local machine, even if the interface configuration changes.

[nahiluhmot]

@willglynn Interesting, I must admit that I'm at best a journeyman with iptables. Going to test this out to make sure it works then merge it.

[nahiluhmot]

@willglynn Looks like you'll have to rebase onto master.

[willglynn]

Rebased.

[nahiluhmot]

This looks good, merging.