added reserved teleport channels

With this change, teleporter tube channels that start with a name followed by a colon or semicolon are reserved for that player: channels with a colon can only be used by the owner player while other players can use a semicolon channel as long as the tube is not set to receive.

For instance, if my name is hdastwb, then I will be the only one allowed to set a teleporter tube to something like 'hdastwb:stuff'; anyone else trying to set a teleporter channel to a name starting with 'hdastwb:' would be denied access. If I defined a channel 'hdastwb;mail', people would be able to send me stuff but would not be able to intercept things from me.

There are a couple categories of people who cannot use this system:

1. people with empty usernames (':public default:cobble' is a valid public channel name; since it seems to be impossible to join with an empty username anyway, this can be used to provide public channels containing colons or semicolons)
2. people whose names contain a colon or semicolon (I've never met any, but sorry to any that exist; they'll still be able to use teleporter tubes but they will not be able to use their own private channels because of the way the notation works)