// 2-byte sequences:
// IAC SE (240) End of subnegotiation parameters
// IAC NOP (241) No operation
// IAC DM (242) Data Mark (used with TCP urgent pointer)
// IAC BRK (243) Break (simulate serial line break) - perhaps tcsendbreak(master_ptyfd)? [untested]
// IAC IP (244) Interrupt Process ("send SIGINT to foreground process group a-la ^C")
// IAC AO (245) Abort Output
// IAC AYT (246) Are You There
// IAC EC (247) Erase Character
// IAC EL (248) Erase Line
// IAC GA (249) Go Ahead
// IAC IAC (255) I'm sending literal byte 255
// 3-byte sequences:
// IAC WILL <option> (251) sender declares willingness to enable option
// IAC WONT <option> (252) sender refuses / disables option
// IAC DO <option> (253) sender requests peer to send option, or enable option on peer's side
// IAC DONT <option> (254) sender requests peer to NOT send option, or disable option on peer's side
// Example:
// IAC DO 31 (request NAWS)
// IAC WILL 24 (offer TERMINAL-TYPE)
//
// Variable length: subnegotiation (250)
// IAC SB <option> <data...> IAC SE
// option = single byte
// data = zero or more bytes
// Subnegotiation can not be nested, and can not have 2- and 3-byte commands inside.
// Inside subnegotiation data, IAC IAC represents literal 255.
// An IAC byte inside SB:
//  IAC IAC: literal byte 255
//  IAC SE: end of subnegotiation
//  IAC <anything else>: protocol error
// Common options that use subnegotiation:
// TERMINAL-TYPE (24)
//  IAC SB 24 SEND(1) IAC SE
//  IAC SB 24 IS(0) "xterm-256color" IAC SE
// NAWS - Negotiate About Window Size (31)
//  IAC SB 31 <width_hi> <width_lo> <height_hi> <height_lo> IAC SE
//  width and height are 16-bit big endian. HOWTO:
//  Server: IAC DO NAWS
//  Client: IAC WILL NAWS
//  Client: IAC SB NAWS <w> <h> IAC SE
//  ...sometime later: client's window is resized by user...
//  Client: IAC SB NAWS <w> <h> IAC SE
// TERMINAL-SPEED (32)
//  IAC SB 32 IS(0) "rx,tx" IAC SE
// REMOTE-FLOW-CONTROL (33)
// LINEMODE (34)
// X-DISPLAY-LOCATION (35)
// ENVIRON (36)
// AUTHENTICATION (37)
// ENCRYPT (38)
// NEW-ENVIRON (39)
// COM-PORT-OPTION (44)