External IP address port range is exclusive of last port

OPTE accepts a `std::ops::Range` as its port-range for source NAT external IPs. That's exclusive of the last port, which is a bug in OPTE (not yet filed). When that's resolved, change the port-range in `sled-agent/src/opte/opte.rs` when creating the `SNatCfg` to include that last port and remove the saturating add call there. Links to follow.