Method in TelnetOptionHandler to mark sub-negotiation as 'used'


Currently TelnetStream decided whether a SubNegotiation command is 'used' depending on whether one of the added TelnetOptionHandlers has called AddResponseSubNegotiation.  When not apparently handled' it logs a diagnostic message ("Discarding SubNegotiation...").  In some cases, e.g. StatusReceiveTelnetHandler receiving a status report, no response is possible but the command has been handled.  We should add a method like 'MarkSubNegotiationUsed' to handle this situation.
(Or should we be passing an EventArgs to such methods and thus there would be a method on it).