Update redirect-gateway example.

Update redirect-gateway example.

- Correct some mistakes in the example output of "route -n".
- One must add a route to a gateway before one can add gateway routes.
- Make it more clear one must also set up masquerading or other ways of routing
  on the server, enable forwarding, and possibly modify firewall rules.
- Add a note about the effect of router and switch mode on redirecting ther
  gateway.