Option to enable/disable RTS/CTS (hardware) flow control from Erlang. Hardware flow control must be disabled to interact with the Arduino serial port. The way the code is structured, options are disabled by default and activated by sending a message. They cannot be disabled afterwards. This means changing the port behaviour to disable flow control by default. Consequently, code must explicitly enable flow control; updated slip/terminal examples (not tested). Also, the port originally set flow control every time the speed changed.