Allow /unregister [username] in-game

[oplexz]

Either what the title says, or ignore when someone attempts to do /unregister [nickname] in-game, since it causes the player executing the command to be unregistered.

P.S. as far as I'm concerned, there's no information about being able to /unregister other players in the help entry, so making it that /unregister [username] works both in-game and in console, and adding it to the /help entry would solve this issue.