chardev: Basic support for TN3270

chardev: Basic support for TN3270

This introduces basic support for TN3270, which needs to negotiate
three Telnet options during handshake:
  - End of Record
  - Binary Transmission
  - Terminal-Type

As a basic implementation, this simply ignores NOP and Interrupt
Process(IP) commands. More work should be done for them later.

For more details, please refer to RFC 854 and 1576.

Signed-off-by: Jing Liu <[email protected]>
Signed-off-by: Yang Chen <[email protected]>
Reviewed-by: QingFeng Hao <[email protected]>
Acked-by: Dong Jia Shi <[email protected]>
Signed-off-by: Cornelia Huck <[email protected]>
Reviewed-by: Marc-André Lureau <[email protected]>