Add PermitListen support for port 0

[GuyLewin]

As part of a project I'm working on I want to limit an SSH user to only be able to create remote forwards with a randomly binded port (0). Current implementation doesn't let the user permit port=0 on remote forwards, even though it's a valid input in remote forwards (not in local forwards). With this PR the context of what flow is being passed into the port validation, taking it into account.

[GuyLewin]

@djmdjm is there anything I should fix?

[GuyLewin]

@djmdjm bump :)