#557 IAC IAC should always become 0xFF in Telnet emulation

[Sgeo Sgeo]

For nullmodem and softmodem's telnet emulation, the current behavior is that incoming 0xFF 0xFF gets converted into a single 0xFF, only if the server has previously indicated that it wants to (or will?) send in binary.

My understanding of Telnet is that IAC IAC (0xFF 0xFF) should always be 0xFF, regardless of binary mode.

I encountered this trying to get old software that does not recognize telnet to connect to a modern service that implements telnet by converting 0xFF into 0xFF 0xFF and 0x0D into 0x0D 0x00.

I'm not entirely confident if the telnet emulator is supposed to cover this, but I'm also unsure why else the emulation exists.