Implement a simple serial TCP port server using C.  At
startup, the program should open a serial port device (default
/devb/ttyS0) and set the baud rate (default 9600 ).   It
should also create a TCP socket and bind it to a port (default
10000).  The program should then listen for connections on the
TCP socket.

While a client is connected any data received from the client
should be written out to the serial port, and any data
received from the serial port should be written back to the
client.

USDD will provide a shell account on a Linux development
server with 2 serial ports. The serial ports will be connected
together with a null modem cable.  

(Explain what a null modem cable is).

A terminal emulator program will be available for interacting
with the serial connection.  USDD can install a text editor
other than emacs, vi, or nano if requested. The program must
be compiled using GCC and cannot be linked to any non-libc
libraries other than libevent.  Use of Google, man pages, and
other reference material is permitted and encouraged.

- only 1 TCP connection can be active at a time, subsequent
      connection requests should be refused.

- The program's main loop use I/O multiplexing, either with
      the select/poll family of functions or libevent's
      event_dispatch function as the main loop.

- The program should print messages to the console for events
      like "connection accepted" or "closed", "number of bytes
      sent or received" from the TCP socket or serial port, and
      any error messages.

- Allow the user to specify the serial port device, the baud
      rate, and the TCP port number using command line arguments
- Use libevent instead of select/poll
- Use non-blocking I/O (O_NONBLOCK) and handle buffering and
      partial writes (EAGAIN).

Whenever the program receives an STX (0x02) byte over the TCP
connection, assert the RTS line instead of writing it out to
the serial port.  Similarly, when it receives an ETX (0x03)
byte, de-assert the RTS line.