Firewall: DNat: Display effective port when local-port is omitted

[TomWalraven]

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Describe the problem

Currently when selecting any/skipping local-port field, it's unclear which port is effective.

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Describe the proposed solution

Display the selected destination port in the local-port field. Local-port will only display "*" if both destination port and local-port are omitted.

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Related issue
Closes: #9677