[nsp] PIX 535 stateful failover

[fingers]

> (There are some bizarre restrictions regarding interface naming and
> security levels I've run across, but if it tries to stop you, you can
> always call the inside interface something different and make it SL99
> instead of SL100, which would have exactly the same effect anyway)

in some previous versions of code (early 6*), if you called 'inside' by
any other name, you weren't able to run services like dhcp against it
anymore. afaik they fixed it in later versions