telnet: add disabled code to emit EC and IP

telnet: add disabled code to emit EC and IP

> I'm trying to connect to a Korenix 3005 switch through telnet
> for management purposes, and all is well except for the backspace character
> - seems like my switch doesn't handle it too well and instead of erasing
> the last character all it does is print some garbage to the screen.
> I've had the same issue before while using putty, but saw a solution that
> suggests to enable "Telnet special commands" in the options menu, and it
> worked.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>