todo

网络端口的服务程序

o  Switch over to a multi-process or multi-threaded model so
   that TCSADRAIN can be used without blocking all threads.

o  Actually implement PAM-based authentication

o  Implement a <FLOW xxx> command to select different flow
   control options on the port.  Most urgently needed is
   XON/XOFF.

o  Add -p command line option to select TCP port to listen on.
   This should override the setting in the config file.